Driving Economically - 56mph Myth??

[jdm01]

I remember reading about a recommendation that driving at 56mph will give the max performance efficiency in terms of driving a motor car.

It was suggested that this applied to all cars no matter what the size of engine.

I am of the opinion that this is an urban myth, can someone prove me wrong? This figure has stuck in my head and I always feel myself trying to get close to the 56mph on motorways (albeit in a 3L diesel engine & I still believe it is a false figure).

If it is a myth, has anyone got any way of calculating the max fuel efficiency from a car.

J

[Lorian]

I always feel myself trying to get close to the 56mph on motorways (albeit in a 3L diesel engine & I still believe it is a false figure).

Oh, that's you eh?

I'm suprised a 3 Litre car doesn't have a fuel computer to tell you the mpg.

[balsingh]

I suppose it is sort of a myth .... i always thought the best economy was acheived when cruising at the lowest possible comfortable speed in the highest possible gear. By comfortable, I mean not so low that it almost demands chaging down a gear but a speed that its well settled at lowish-rpms. For most cars, this is around 50mph so I think 56mph was used to cover the fact that some cars have 3 cylinder 1 litre engines and others have 3 litre diesels with 6 speed boxes.

[lellie]

56mph is also 90 kmph - which is a common speed limit abroad.

[AMO]

It's a myth - sort of. It's different in each car depending on the number of gears and rpms (diesel/petrol).

However, I believe that even though most cars might deviate from 56mph, its never going to be too far off, as once you go into the higher speeds you have to calculate adverse resistance from driving into the wind etc.

AMO

[jellycat40]

one of the questions for my daughters practice theory test was "what is the percentage of fuel saved at 50mph as opposed to 70mph"? the answer was 70% so the DVLA must have some reason for putting this in.

The slower you cruise at the better economy you get providing you are in the highest gear you can be in for the speed.

Louise

[philhuff]

Similar to all of the above, the very rough ideal speed will be around 35mph in as high a gear as the car manage comforably in.

Beyond that, wind resistance builds and you spend a lot more energy cutting through the air. Resistance increases at a rate square to the speed, so doubling your speed means you'll need four times the energy to get through the air.

The above is very generalised. Please don't pick too many holes in it, or I'll be forced to come back and bore you all to death :)

[Idiophreak]

This figure has stuck in my head and I always feel myself trying to get close to the 56mph on motorways (albeit in a 3L diesel engine & I still believe it is a false figure).


Always wondered who these people are in big cars holding me up on the way to work every day - in a way it's nice to finally meet one.

As a hint, if you're so concerned about economy, just get a smaller engine and GET A MOVE ON!

[The_Biff]

This (56mph) was the national speed limit in the USA in the 1970's when the petrol crisis was on. This was an average mpg for all US cars.

[abarthman]

This (56mph) was the national speed limit in the USA in the 1970's when the petrol crisis was on. This was an average mpg for all US cars.The US speed limit was 55mph. I don't understand the second sentence.

I think lellie is correct.

[ollyk]

This (56mph) was the national speed limit in the USA in the 1970's when the petrol crisis was on. This was an average mpg for all US cars.

A US car that does 56mpg? Pull the other one :rotfl:

[jdm01]

Some nice responses! Some nice calculations, however I am still not convinced.

I would reckon a number of factors exist which many have touched on. I believe someone hit it on the head when they mentioned you would need to drive at the lowest comfortable revs in the highest gear which would be a lot different dependent on the size of the engine and the weight of the car.

It would be good to see true graphs for each car out there. @albertross - I think these graphs used in logbooks are the same graph, they always peak at around 56mph - you never ever see one going over 70mph (which I'm sure some cars would have).

I always reckoned that air resistance (albeit a contributing factor) does lessen as an negative force (at normal speeds) as the years pass and engines become more powerful & cars become more heavier.

Thanks to everyone for your responses.

J

[Murphy_The_Cat]

jdm01, just out of curiousity is your car an auto, and if so, is it a 5/6/7 speed box. Nowt to do with your question, just being nosey :D

Back to your question, I don't know if 56 mph is the 'ultimate' economy speed, but if I keep to it in my car, the economy is very, very good !

MTC http://ofuabduction.com/images/alex/HissyClaw.gif http://i42.photobucket.com/albums/e347/Murphy2006/MTCEnglish.gif

[bunking_off]

56mph is the speed that manufacturers are compelled to publish MPG figures for. Now, put yourself in the position of a manufacturer - if you were designing your car, wouldn't you sorta set it up such that it gave the most flattering value at that speed, given it's mandatory to state the figure in any advert? Perhaps that would answer the question - may vary from car to car and 56mph may not be the most economical, but it won't be far off...

[MarkyMarkD]

Bunking_off, you are out of date. Manufacturers now publish urban and extra-urban MPG figures, which are based on different driving cycles. The old system of publishing MPG at fixed speeds (56 mph and 75 mph) was clearly stupid for the reason you pointed out, which is why it's been changed.

Obviously, manufacturers still optimise their cars to do well on the new driving cycles, but it's harder for them to manipulate things.

Less powerful cars with lower maximum speeds will have a far lower optimal speed than more powerful cars with higher maximum speeds, so to generalise that 56mph is optimal is false.

Wind resistance makes a huge difference (and it's a squared relationship i.e. the wind resistance increases four times when your speed doubles) so for any car the optimal speed won't be hugely high. But for some cars it will be way lower than 56mph.

In any case, it isn't speed but throttle that makes a difference. Obviously you can cruise at 60mph down a steep hill using (virtually or) no fuel at all, or climb up a mountain at 15mph using huge amounts of fuel.

[bunking_off]

Ah, makes sense.

Must admit that my motor - 4.2 supercharged V8 so I guess falling into your definition of more powerful (!) - gives a lot better fuel consumption figures at 80mph than it does at 70mph. Not that I ever exceed the limit, of course...

[jdm01]

jdm01, just out of curiousity is your car an auto, and if so, is it a 5/6/7 speed box. Nowt to do with your question, just being nosey :D

Kia Sedona crtdi 7seater (I have a big family!) - Only 5speed

Cheers...J

[Wig]

one of the questions for my daughters practice theory test was "what is the percentage of fuel saved at 50mph as opposed to 70mph"? the answer was 70% so the DVLA must have some reason for putting this in.

The slower you cruise at the better economy you get providing you are in the highest gear you can be in for the speed.

That doesn't make sense, maybe you were just using random figures to illustrate the point?

If you look at the graph provided in the thread
70 mph 25mpg = 100 miles = 18.18 litres
50 mph 30mpg = 100 miles = 15.15 litres

Another examples would be
70 mph 33mpg = 100 miles = 13.64 litres
50 mph 45mpg = 100 miles = 10.1 litres

So in the first example the percentage of fuel saved is 17%
In the second example (my figures) % fuel saved is 26%

[tomstickland]

Drag power scales with speed cubed, so low speed will require less engine power and therefore fuel consumption. However, engine efficiency is rpm dependant (low rpm is inefficient due to gas filling efficiency , high rpm is high mechanical frictional losses). Therefore the peak efficiency would be expected to occur with the engine at optimum rpm and the car travelling as slowly as possible. So about 15mph in 1st gear.

Thinking about it a bit more, fuel economy depends on load and power, so 15mph in 1st gear probably is not putting enough load on the engine for peak efficiency.

The 56mph thing is a EU fuel economy test, so manufacturers tune their ECUs to give good fuel economy at this speed.I couldn't drive on a motorway at 56mph in a 3L engined car!


If fuel economy scales with speed squared I'd expect 50vs70mph to increas mpg by 50%. (Power requirement scales with speed cubed, but miles travelled per unit time scales with speed, so energy used per mile scales with speed sqaured).

[Murphy_The_Cat]

The 56mph thing is a EU fuel economy test, so manufacturers tune their ECUs to give good fuel economy at this speed.I couldn't drive on a motorway at 56mph in a 3L engined car!



of course you could, just set the cruise and away you go.

whether you would ever WANT to do this though is another ketle of fish entirely.

MTC http://ofuabduction.com/images/alex/HissyClaw.gif http://i42.photobucket.com/albums/e347/Murphy2006/MTCEnglish.gif

[jonuk7]

Not sure if 56 mph is the ideal speed, but your fuel economy has nothing to do with speed its on the RPM, so the lower you can keep the revs the better mpg you'll get, if you could drive at 100mph at 3000rpm youd get pretty much the same as at 50mph at 3000rpm obviously the integral factors then become, road condition, weather/wind etc. Not sure about other cars but i have an 03 Astra SXI which has a fuel limiter which kicks in when you are not touching the accelerator and/or braking.

Also just to add upto 50mph do not use A/C as having your windows down is more econimical and then above 50mph close your windows and use A/C.

[Murphy_The_Cat]

Kia Sedona crtdi 7seater (I have a big family!) - Only 5speed

Cheers...J

jdm01, not the car that I was thinking about - I was just about to enter full banter mode with you :D

MTC http://ofuabduction.com/images/alex/HissyClaw.gif http://i42.photobucket.com/albums/e347/Murphy2006/MTCEnglish.gif

[tomstickland]

Not sure if 56 mph is the ideal speed, but your fuel economy has nothing to do with speed its on the RPM, so the lower you can keep the revs the better mpg you'll get, if you could drive at 100mph at 3000rpm youd get pretty much the same as at 50mph at 3000rpm

That is not correct.

Fuel consumption is a function of the power output required and engine efficiency.

Engine output power required is mainly dependant on the aerodynamic drag forces that have to be overcome. The drag power scales with speed cubed, so 100mph requires 8 times as much power as 50mph.

If your statement was correct then driving up a 1:10 hill all day at 3000rpm would require the same amount of fuel as driving down a 1:10 hill at 3000rpm.

Or driving into a 20mph headwind at 3000rpm would use the same amount of fuel as driving with a 20mph trailing wind at 3000rpm.

You know that if a car comes to a hill then you have to open the throttle to maintain the same speed. This allows a greater air flow rate into the engine for the same rpm. Petrol engines need to run close to the ideal ratio to operate, so more air means more fuel has to be added.

The drag forces are what ultimately limit the top speed of cars.

[Idiophreak]

I'd just like to add that all of the math/ideas above are all very theoretical, and few of them have much to do with fuel economy in the real world.

If you drive at 56mph on the motorway the following will eventually happen:

1) A lorry will catch up to you.
2) The lorry will move out to overtake.
3) The holier-than-thou middle lane hogger doing their steady 65 mph will move to overtake the lorry, probably without bothering to check their mirrors. (afterall, they're doing 65mph, nobody could possibly be wanting to go faster)
4) The BMW driver behing h-t-t who was previously doing 120mph is forced to stamp on his brakes to avoid an accident. The audi driver behind him doing 110mph will do the same, then the 100mph white van behind him, then the 90mph vectra.
5) The newly trained and enthusiastic 80mph mini cooper driver will see the brake lights ahead, brake early and put on their hazzard warning lights. People behind will then slow further and do likewise.

The result? Everyone sits in a massive traffic jam and guzzles fuel by the gallon just because 1 person couldn't be bothered to drive at a speed appropriate for the road.

[tomstickland]

Whilst I agree that all of the above is annoying, it doesn't lead to guzzling fuel by the gallon at a greater rate than they were when they were travelling faster. The act of slowing and then speeding back up does.

In rough order of efficiency
-driving slowly at a constant speed
-driving quickly at a constant speed
-wildly fluctuating speeds

few of them have much to do with fuel economy in the real world.
Most of them do.

[wdyw]

That is not correct.

Fuel consumption is a function of the power output required and engine efficiency.

Engine output power required is mainly dependant on the aerodynamic drag forces that have to be overcome. The drag power scales with speed cubed, so 100mph requires 8 times as much power as 50mph.



Tom has it just about spot on except if you want to get things exact you also need to add in rolling friction that is usually a lot smaller than the aerodynamic drag....and of course while the rolling drag would stay constant if you just considered the mass of the car, it does vary a bit with car speed, but still fairly insignificant.

[MarkyMarkD]

I'd just like to add that all of the math/ideas above are all very theoretical, and few of them have much to do with fuel economy in the real world.

If you drive at 56mph on the motorway the following will eventually happen:

1) A lorry will catch up to you.
2) The lorry will move out to overtake.
3) The holier-than-thou middle lane hogger doing their steady 65 mph will move to overtake the lorry, probably without bothering to check their mirrors. (afterall, they're doing 65mph, nobody could possibly be wanting to go faster)
4) The BMW driver behing h-t-t who was previously doing 120mph is forced to stamp on his brakes to avoid an accident. The audi driver behind him doing 110mph will do the same, then the 100mph white van behind him, then the 90mph vectra.
5) The newly trained and enthusiastic 80mph mini cooper driver will see the brake lights ahead, brake early and put on their hazzard warning lights. People behind will then slow further and do likewise.

The result? Everyone sits in a massive traffic jam and guzzles fuel by the gallon just because 1 person couldn't be bothered to drive at a speed appropriate for the road.As most lorries are restricted to 56mph, this would only happen because the first driver was driving at an indicated 56mph (i.e. c.50mph) rather than a true 56mph, but I like your point. :)

[wolvoman]

Whilst I agree that all of the above is annoying, it doesn't lead to guzzling fuel by the gallon at a greater rate than they were when they were travelling faster. The act of slowing and then speeding back up does.

In rough order of efficiency
-driving slowly at a constant speed
-driving quickly at a constant speed
-wildly fluctuating speeds

Except you've left off the most inefficient of all: stationary with the engine running. Which is what happens in a traffic jam as the previous poster mentioned.

[110frankie]

a number of different agencies independently came up with 56 mph as the most fuel efficient cruising speed - this was in the 1970s when fuel costs were becoming very important all of a sudden - for all vehicles, regardless of engine size and vehicle shape.
Quite obviously this would have to be an average.
Governments dived on this figure as a holy grail - in the States they hit on 55 (the "double nickel") as a limit (which led to the rise in CB radios amongst truckers as they couldn't earn a living driving hundreds of miles at 55 mph) and in the UK they went for 70mph as a motorway limit... (no, we - I am a motoring journalist - didn't understand it at the time either, but that is the UK Government for you).
So we had an average, which means it was hardly a good idea for any vehicle, and that was 30 years ago.
Since that time engines have changed and vehicle body shapes have changed (and my hair has gone grey) but politicians and "experts" just stare at that 56 number and don't change...
the figure might still be 56, or 63, or 48... who knows? No-one's researching it these days... and it would STILL only be an average.

[wdyw]

As most lorries are restricted to 56mph, this would only happen because the first driver was driving at an indicated 56mph (i.e. c.50mph) rather than a true 56mph, but I like your point. :)

Maybe lorry speed is calculated using the same pitot-static tubes used in planes. Then is the 56mph:aTrue speed,an indicated speed,an equivalent speed or a calibrated speed .......arrrgggghhh

Anyway I think most of use agrre that most lorries aren't limited to 56mph on flat roads!

[tomstickland]

Except you've left off the most inefficient of all: stationary with the engine running. Which is what happens in a traffic jam as the previous poster mentioned.
Except what was described isn't enough to cause a full on traffic jam. Also, the other drivers should help smooth out the speed transition by anticipating. I read some research that said about 50% of drivers exacerbated problems like this by braking hard and late rather than smoothing it out.

[iltisman]

I have just finished from a job that was a 282 round trip and over 6years I perfected economical driving, freewheel when possible, anticipate so no breaking is required, let speed bleed off up hill, gain speed on non freewheelable descents,use gps to display arrival time and slow to achieve on time arrival and finally dont be intimidated into burning more fuel by non frugal drivers. Oh I forgot over inflate tyres by 10%.

[achtunglady]

This figure has stuck in my head and I always feel myself trying to get close to the 56mph on motorways (albeit in a 3L diesel engine & I still believe it is a false figure).


Why are you not driving at the speed limit on the motorway, its 70mph in all lanes except for lorries, which cannot use the outside lane. One thing that peeves my other half off is an idiot going at 50 mph in the middle lane, sitting in the blind spot of his lorry, when he's trying to get somewhere to take a break which he has to do by law. The police or traffic commisioner will not take the excuse of "i was stuck in traffic" or "i couldnt overtake because some idiot was lane hogging in the middle lane" and are very heavy when it comes down to dishing out fines and revoking licenses. Your car can do 70 so do it!! If you dont want to do 70 then stick to the A roads

[wdyw]

One thing that peeves my other half off is an idiot going at 50 mph in the middle lane, sitting in the blind spot of his lorry, when he's trying to get somewhere to take a break which he has to do by law.

If the car doing 50mph is in your fellas blind spot then surely he is behind his lorry and hence not impeding him?? - unless you mean he is in the blind spot just in front of your fellas lorry and in the next lane across, and of course its a blind spot as your fellas lorry is a foot behind the lorry in front trying to save a penny of fuel.

Ban all HGV's between 7am - 7pm on motorways. That's the first thing I would do if I had my way.

[tomstickland]

gps to display arrival time and slow to achieve on time arrival and finally dont be intimidated into burning more fuel by non frugal drivers
A fascinating insight into a reason why some people choose to drive down straight A roads at 40mph?

[martindow]

Definitely not advisable but presumably driving close to the back of a lorry would remove a lot of wind resistance and aid economy. I know that some cycling speed records were made using a wooden track laid between railway lines and the cyclist following a train. The last carriage was fitted with a sort of cowl that enclosed the cyclist.

[achtunglady]

If the car doing 50mph is in your fellas blind spot then surely he is behind his lorry and hence not impeding him?? - unless you mean he is in the blind spot just in front of your fellas lorry and in the next lane across, and of course its a blind spot as your fellas lorry is a foot behind the lorry in front trying to save a penny of fuel.

Ban all HGV's between 7am - 7pm on motorways. That's the first thing I would do if I had my way.

Yes I do mean the blind spot in the middle lane, and if hgvs were banned on motorways during the days, how do you think any goods will be transported?
There would be no tankers/food deliveries/mail on the roads. Not all premises are open 24 hours a day to accept deliveries at night, so shops will have to pay out more for staff to work during the night, thus putting the cost up for the consumer.

And this great debacle about foreign lorries side swiping cars as they overtake, cars have an extra lane to drive in, lorries can only use the middle and inside lane, so really theres no excuse for this happening, only happens if you hog the middle lane and are not aware of the traffic around you and are not anticipating other road users maneouvres and by not going at the speed limit.

[tomstickland]

Lorries on motorways are not a big problem IMO. People complain about them as a distraction from their own ineptitude.

Regarding being sideswiped, whenever you overtake something you should be looking to see if it might have cause to pull out and look in the mirror to see what's coming up in the outside lane. It's called anticipation.

Talking of which....one day I was accelerating down a slip road onto the motorway. There was a lorry in the LH lane and a car behind me in the middle lane. Outside lane was empty. I pulled out round the lorry into the middle lane. Car had plenty of time and space to sort things out. What did they do? Yes, flash me.

[tomstickland]

You should anticipate anything that could reasonably happen. In congested traffic, with three lanes moving at a similar speed then there's not much you can do and you'd hope that a lorry would take its time pulling out, but with when the road is moving at higher speed then you should anticipate a lorry pulling out so you can act swiftly if it does.

From my observations, lorries are not a problem.

[tomstickland]

I shouldn't have to check that a pedestrian isn't going to walk out into the road, but I do. Your adding something to cover for the possibility of someone else making a mistake.

[tomstickland]

Well, I don't like being level with a lorry for a long period of time. I tend to hang back and then speed up past them to reduce the amount of time in the danger zone.

[MickKnipfler]

Just get a 3 cylinder diesel Hyundai Getz and rev the b0ll0x off it! 80mph 80mpg, easy!

[SeriouslyStressed]

just buy an economical scooter :rotfl: something around the 100mpg mark:D

[110frankie]

I'm happy enough with my Range Rover with LPG.
Cost to buy £1500
cost to run 36p a litre

now that's an MSE vehicle !

[SeriouslyStressed]

ah.. range rover.. but you dont mention the actual costs...of running...

tyres, brake pads, disc's, spark plugs, bearings, oil, filters, exhaust, suspension parts etc, etc... it'll all be to buy at some point along the way as is insurance, tax, mot fee etc...

these are the hidden costs.

Generally, the older the vehicle, the more it will need doing to it.
It is negligent to leave something that needs doing until MOT time as it might be that the vehicle is unroadworthy/unsafe in the meantime.

Having said all that... LPG seems a great option if you have LPG stockists on your regular routes out and about.

[kittiwoz]

Generally, the older the vehicle, the more it will need doing to it

Not exactly. The failure probability density curve for a car, as with many things, is bathtub shaped. This means there is an initial period whith a high probability of failure (refered to as infant mortality) which gradually levels out (typically by three years). This is what warranties are designed to cover. Then there is a long period where the probability of failure is constant which lasts up til about 100,000 miles. Failures in this period will be largely consumables. After that the probability of failure will increase as the vehicle nears end of life. Between three years and 100,000 miles the relaibility is the same. However obviously the older the car is the sooner it will reach 100,000 miles and start to need more work.

[tomstickland]

I've only been under 100K in any of my cars for a few months; in 10 years I've probably spent 98% of my time with +100K cars. I've got a 178K BMW at the moment and spend virtually nothing on it.

[kittiwoz]

Well that's supposed to describe a typical curve but I don't know how old the figures it is based on are and it does vary between types of car. I'd expect a BMW to last well because they are known for solid build.

My dad normally has cars that have done over 100,000 miles, many of them have been older than I am (24). While he spends less on them than many people with much newer cars because he does his own maintenance he does put quite a bit of time into them. Whether around 100,000 miles or later cars certainly do reach a point where they become less reliable.

But really my point was that there is a long period of time where the reliability is constant and almost all the things mentioned by SeriouslyStressed are consumables so the likelihood of them needsing replacement is largely not dependent on the age of the vehicle. So it is not true to say that, generally, the older the vehicle, the more it will need doing to it and in fact a car that is, say, 5 years old will normally be more reliable than the same car was when 6 months old.

[wolvoman]

Back on topic I was playing about with the trip computer in my car last weekend at night so the motorway was very quiet and I wasn't holding anyone up.
I'd reset the trip computer and drive for 1 mile at each of several speeds and note the average mpg over that mile. The speeds are all GPS speeds (I have a gadget fitted to the car).

45mph - 43.7 mpg
48mph - 43.9 mpg
50mph - 45.7 mpg
53mph - 46.3 mpg
55mph - 47.9 mpg
58mph - 49.0 mpg
60mph - 48.8 mpg
63mph - 48.0 mpg
65mph - 46.0 mpg
70mph - 41.3 mpg
80mph - 33.0 mpg
90mph - 26.5 mpg
Won't go any higher!

Unscientific I know but it shows a pattern.
This was in a 3.2 litre Porsche so not the most economical example.

[tomstickland]

My dad normally has cars that have done over 100,000 miles, many of them have been older than I am (24). While he spends less on them than many people with much newer cars because he does his own maintenance he does put quite a bit of time into them. Whether around 100,000 miles or later cars certainly do reach a point where they become less reliable.
I reckon it's about 200K that a car is getting to the point of excess wear.

[Madiba]

The most economical speed for any car is its maximum speed with no gas.

[tomstickland]

What do you mean by "no gas"? Do you mean with the engine turned off, or foot off accelerator?

[Madiba]

Of course, I mean with the foot off the fast pedal.

Driving around with the engine off would be silly. :D

[tomstickland]

Well, I'd take issue with your definition. The most efficient operating point for most engines occurs under some load and somewhere in the 2000-3000rpm range. Driving round "off the gas" in most situations will not be running the engine at peak efficiency. I agree that "foot to the floor" is the least efficient situation.

[tinker1]

So you want to go for best mpg. This is a bit dangerous but your mpg will shoot up.
Get yourself behind a lorry at 60mph and tailgate him so you are sucked along behind him with your foot just touching the throttle. Bl..dy dangerous but it works a treat for a free ride.

[Stephen Leak]

According to the MPG readings (both spot and average) from the trip computer in my 10-year-old 1.8 petrol Mk.2 Mondeo, the optimum seems to be at about 50mph in 5th gear.

[judderman62]

jdm01, just out of curiousity is your car an auto, and if so, is it a 5/6/7 speed box. Nowt to do with your question, just being nosey :D

Back to your question, I don't know if 56 mph is the 'ultimate' economy speed, but if I keep to it in my car, the economy is very, very good !

MTC http://ofuabduction.com/images/alex/HissyClaw.gif http://i42.photobucket.com/albums/e347/Murphy2006/MTCEnglish.gif

so should I stop driving at 80 mph then

[markymoo]

i average about 48mpg on the motorway at around 70-75mph...
i average about 45mpg on the motorway at a slightly higher speed (if i were allowed to do it)

i have no idea what my average mpg would be at 56mph, but i know i'd be out of a job if i couldn't drive at the speed limit...
cars who drive at 56mph on the motorway should only be allowd on at weekends, so not to hold other people up who have to be at 6 different places through out the day...

[vikingaero]

56mph was an arbitrary limit chosen to test vehicles because it equates to 90kph.

With all the theories here, there is no set formula for vehicles because each car has different drag, number of gears, engine size, engine types etc etc. I'm surprised that no-one has mentioned gearing. You can have a 6 or 7 speed gearbox but the way the manufacturer has defined the gears will affect mpg.

On A roads where you cannot overtake the 40mph dwardler I find that I get terrific mpg!

[movilogo]

The most optimum driving speed depends on each engine's power torque characteristics. There can never be a single optimum speed for all cars!

[thescouselander]

Well that's supposed to describe a typical curve but I don't know how old the figures it is based on are and it does vary between types of car. I'd expect a BMW to last well because they are known for solid build.




Obviously you dont frequent any BMW message boards then. BMWs are anything but reliable, just ask someone who has one of the recent diesel powered models whos plastic inlet manifold has disintegrated and destroyed the engine. This has happened on cars that are well under the 100,000 mile mark!

[LandyAndy]

The most optimum driving speed depends on each engine's power torque characteristics. There can never be a single optimum speed for all cars!

An engine is at its most efficient (ie burning least fuel for the work it is being asked to do) at its torque peak.

This is typically somewhere between 1750 rpm and 2500 rpm for a diesel and 2500 rpm and 4500 rpm for a petrol.

Combine that with typical gearing in top gear 25 - 30 mph per 1000 rpm and you get an idea of what is likely to be the most efficient speed to drive at.

However take my Mazda RX-8 PZ. Peak torque is at 5500 rpm. Gearing in 6th is 20 mph per 1000 rpm. So most efficient speed in 6th gear is 110mph:D

I was just driving efficiently , officer.:j

[boyse7en]

>So most efficient speed in 6th gear is 110mph

No, the most effiecient use of fuel in a your engine when static (i.e. if it was on a test bench) would be @ 5500RPM - that is it will give the best ratio of power to the amount of fuel required. This does not mean that it is the speed at which it would use the least amount of fuel.

I know it was a tongue-in-cheek comment, but don't get everyone to confuse efficiency with economy.

Plus, there is no consideration of aerodynamic nor rolling resistances, but there has been enough mention of those already...

[movilogo]

An engine is at its most efficient (ie burning least fuel for the work it is being asked to do) at its torque peak.

I beg to differ. Torque peak is the point where it has highest hauling power, however, you don't always need the highest hauling power!

Why?

Because when a car is run, it has to overcome these resistances = friction, gradient (zero on level ground) and air drag.

The engine produces torque and which manifests as power. Power = Torque x Revolution x [some constant]

Now, as soon as the power produced by engine is more than the power required to overcome friction + air drag, the car will run easily. Usually, this happens at an RPM much less than torque peak RPM (because of the gear) - which is usually quite high [unless it is a diesel or petrol engine with Variable Valve Timing].

If you shift up to higher gears at low speed, the engine will often jerk [since torque is less at low RPM], however, bigger engine you have, sooner you can shift up [because of high torque].

In most engines, lowest fuel consumption tends to occur at 2000-3000 RPM. But this can vary depending on several factors.

The gearbox simply acts as torque multiplier - nothing else, that's why we never need to wait till the redline to change gears. In electric motors, torque is constant at any RPM. So, electric cars don't have gears!

[movilogo]

And remember, if your tyre pressure is not correct, you'll burn 10-20% more fuel irrespective of how good you drive or not.

[LandyAndy]

pigeons, pigeons, cat, pigeons, pigeons. :D

[MickKnipfler]

Obviously you dont frequent any BMW message boards then. BMWs are anything but reliable, just ask someone who has one of the recent diesel powered models whos plastic inlet manifold has disintegrated and destroyed the engine. This has happened on cars that are well under the 100,000 mile mark!

www.bmwlemon.com\dontbmwcare.htm (http://www.bmwlemon.com\dontbmwcare.htm)

[harveybobbles]

Think it depends on the car's engine/gearbox combination.

I had an Audi A4 1.8T which was more economical at 65 than it was at 55...
I replaced it with a BMW 528Auto which was at its best doing 84mph.

It's all to do with gear ratios etc...

[MickKnipfler]

I drive a motorhome and actually find it'd most economical arounf 70mph as it get's up more hills without changing down

[shown73]

I'd be interested if someone with one of those whizz-bang fuel computer thingies could run a test on slipstreaming. I used to have a mk3 Zodiac, manual, (remember them?), and travelled about 30 miles on the then new M40 to Oxford, to work. Well, this thing did 18mpg, whether driven briskly or sedately, and I spent ages behind lorries trying to improve this, but it didn't make a blind bit of difference. It's bothered me ever since, because in theory it should have. Maybe we just weren't travelling fast enough to make a difference, I don't know, but with all the modern widgets, if anyone is seriously bored one day, they could have a go for a few miles, just to see if it works. I wouldn't want to spend a long journey like that, but just as a matter of interest......

[harveybobbles]

I wouldn't want to spend a long journey like that, but just as a matter of interest......

No, but you may spend the next day touching up all the stone chips on your bonnet thru being behind a lorry for so long lol... :rotfl:

[jdm01]

I'd be interested if someone with one of those whizz-bang fuel computer thingies could run a test on slipstreaming. I used to have a mk3 Zodiac, manual, (remember them?), and travelled about 30 miles on the then new M40 to Oxford, to work. Well, this thing did 18mpg, whether driven briskly or sedately, and I spent ages behind lorries trying to improve this, but it didn't make a blind bit of difference. It's bothered me ever since, because in theory it should have. Maybe we just weren't travelling fast enough to make a difference, I don't know, but with all the modern widgets, if anyone is seriously bored one day, they could have a go for a few miles, just to see if it works. I wouldn't want to spend a long journey like that, but just as a matter of interest......


I know it's been a while, but did anyone ever try this? (not that i'm offering)

J

[goldspanners]

i find it hilarious watching someone do this in the rain,with all the spray around, wipers on full pelt,right up the !!!!! of an artic. crazy!

[thescouselander]

I'd be interested if someone with one of those whizz-bang fuel computer thingies could run a test on slipstreaming. I used to have a mk3 Zodiac, manual, (remember them?), and travelled about 30 miles on the then new M40 to Oxford, to work. Well, this thing did 18mpg, whether driven briskly or sedately, and I spent ages behind lorries trying to improve this, but it didn't make a blind bit of difference. It's bothered me ever since, because in theory it should have. Maybe we just weren't travelling fast enough to make a difference, I don't know, but with all the modern widgets, if anyone is seriously bored one day, they could have a go for a few miles, just to see if it works. I wouldn't want to spend a long journey like that, but just as a matter of interest......

Could be interesting. I have a instantaneous MPG needle on my car so it should be immediately obvious how well it works. I have tried this while cycling a few times, works great behind busses but you need to get close - a bit to dangerous really. I'll try it in the car though next time I go on the motorway (leaving a reasonable braking distance).

[vikingaero]

Slipstreaming does work. But to get the full effects you need to be about a foot from the car in front. The driver of the car in front won't be happy and Plod will pull you in an instant.

[thescouselander]

Think it depends on the car's engine/gearbox combination.

I had an Audi A4 1.8T which was more economical at 65 than it was at 55...
I replaced it with a BMW 528Auto which was at its best doing 84mph.

It's all to do with gear ratios etc...

I remember doing some analysis on this during my degree. I cant remember exactly how it worked out but about 55 mph was the best speed to cruise at due to the relationship between drag and distance covered per unit of fuel - this was independent of engine and gearbox efficiencies. Maybe I'll try and dig out the notes next time I'm in the loft.

[jackogoesglobal]

Contrary to previous responses and answers all over the web this is not an arbitrary figure. It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed. Assuming the only other resistance is from the tyres and that this is similar for all cars then the major force that the car has to overcome is the air resistance. Hence 56mph is the most fuel efficient speed ! No wonder then that car manufacturers publish data for their cars at 56mph as it will show them off at their best ! Also it is a happy coincidence that 56mph = 90kmh and therefore the speed limit on the continent. Presumably the limit in the uk was rounded up to 60mph but it could just have easily been 55 . . .

[redux]

It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed.
Absolute nonsense I'm afraid

Of course airflow depends on the shape of the object, and some is laminar at much faster speeds than that.

[AdrianHi]

And remember, if your tyre pressure is not correct, you'll burn 10-20% more fuel irrespective of how good you drive or not.
According to RAC figures
If your pressure is 20% down it increases fuel consumption by 3%.
It's also dangerous, wearing the tyre around 20% faster and you run the risk of a blowout at motorway speeds. 6% of fatal motorway accidents caused by under inflated tyres. 75%-85% (depending on whoes figures your reading) of blow outs cause dby under inflated tyres.

[tomstickland]

Contrary to previous responses and answers all over the web this is not an arbitrary figure. It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed. Assuming the only other resistance is from the tyres and that this is similar for all cars then the major force that the car has to overcome is the air resistance. Hence 56mph is the most fuel efficient speed ! No wonder then that car manufacturers publish data for their cars at 56mph as it will show them off at their best ! Also it is a happy coincidence that 56mph = 90kmh and therefore the speed limit on the continent. Presumably the limit in the uk was rounded up to 60mph but it could just have easily been 55 . . .
Do you have any sources for the claim that it is the speed at which flow becomes turbulent?

[redux]

Do you have any sources for the claim that it is the speed at which flow becomes turbulent?

If the answer contains Reynolds numbers, there's a chance this person knows what they are talking about. Otherwise not

[big gay kirk]

All I xcan say is that AA tests in the 70s showed that a 2 litre Cortina mk 3 did more mpg at 30 than at forty, and that consumption increased as speed increased.. IIRC it jumped horrendously after 60mph... and at seventy it was really bad... I'll try to find the figures....

[big gay kirk]

here we are... from the AA book of money saving motoring.... in 4th gear, here are the consumption figures at constant speed for a Ford Cortina 2000 (thats a Mk 3 by the way....)
10 mph.... couldn't go.. but did 26.3 in 3rd
20mph... 37.7 miles
30mph.. 37.7 miles
40mph... 37 miles
50mph 33.7 miles
60mph... 29.8 miles
70mph... 25.7 miles
80mph... 21 miles
90mph.. 16 miles
in separate tests, they found that traffic conditions affected it thus..
rush hour 17.5 mpg
suburbs 22mpg
less congested 29 miles
winding country roads.. 31mpg
a roads 34mpg
motorway 24.5 mpg..

and driving technique..
n"normal" 26.5mpg
very quiet 32.7 mpg
hard 20mpg

I know its old data, but it just goes to show.. back then 50mph was quoted as the usual "economy" speed.. but it weren't so back then!
(source AA:Money-saving Motoring, London 1974)

[jackogoesglobal]

Yes, this is to do with Reynolds Number. As for 'do I know what I'm talking about', well, I was taught this during my mechanical engineering degree most of which is a distant fog. I'd have to dig out my notes to show you the equation and proof and remind myself the exact details. I'm sure this will be explained in any decent Fluid Mechanics course book. However, what has always stuck in my mind is this strange fact that the change in flow from laminar (smooth) to turbulent flow occurs at the same airspeed and is completely independent of the size and shape of the object (ie car). I'll dig out my notes . . .

[bryanb]

Yes, this is to do with Reynolds Number. As for 'do I know what I'm talking about', well, I was taught this during my mechanical engineering degree most of which is a distant fog. I'd have to dig out my notes to show you the equation and proof and remind myself the exact details. I'm sure this will be explained in any decent Fluid Mechanics course book. However, what has always stuck in my mind is this strange fact that the change in flow from laminar (smooth) to turbulent flow occurs at the same airspeed and is completely independent of the size and shape of the object (ie car). I'll dig out my notes . . .

Been patiently waiting for your notes, Very interested to know how the airspeed is arrived at.

[tomstickland]

I'll do some research into it, but I belive that at all meaningful road speeds the air flow will be turbulent.

[bryanb]

I'll do some research into it, but I belive that at all meaningful road speeds the air flow will be turbulent.

If that's so, what is the point of streamlining? Is it perhaps less turbulent if streamlined?

[tomstickland]

I've got the results from my 1700 mile trip to France and back.
This is based on driving at a near steady speed on motorways, though I did slow on large hills to keep the indicated instantaneous mpg above 30mpg

Mway at 70mph, plus 10 minutes at around that plus 30: 39mpg
Mway at 70mph, being very careful: 42mpg
Cross country and around half a tank on motorway at 70: 39mpg
Mway at 85mph indicated, slowing on big hills: 37mpgs

I'm just finishing off the last tank, which included a long session driving with little regard for fuel economy, I'll update on that soon.

I'm very impressed with these results - this is in a 1990 BMW 318iS (1.8 16v) with 220,000 miles on the clock.

There's about a 5mpg improvement in reducing the speed from 85 to 70mph, which is a 13% improvement for driving around 13% slower.

Just for fun, I worked out that at 70mph it's costing 15p per minute and at 85mph it's 21p/min. Obviouslly at the faster speed less minutes are spent, but as I was sat on the road for 14 hours yesterday, I did wonder what the cost per minute was.

[AdrianHi]

I've got the results from my 1700 mile trip to France and back.
This is based on driving at a near steady speed on motorways, though I did slow on large hills to keep the indicated instantaneous mpg above 30mpg

Mway at 70mph, plus 10 minutes at around that plus 30: 39mpg
Mway at 70mph, being very careful: 42mpg
Cross country and around half a tank on motorway at 70: 39mpg
Mway at 85mph indicated, slowing on big hills: 37mpgs

I'm just finishing off the last tank, which included a long session driving with little regard for fuel economy, I'll update on that soon.

I'm very impressed with these results - this is in a 1990 BMW 318iS (1.8 16v) with 220,000 miles on the clock.

There's about a 5mpg improvement in reducing the speed from 85 to 70mph, which is a 13% improvement for driving around 13% slower.

Just for fun, I worked out that at 70mph it's costing 15p per minute and at 85mph it's 21p/min. Obviouslly at the faster speed less minutes are spent, but as I was sat on the road for 14 hours yesterday, I did wonder what the cost per minute was.
Want to throw your figures into the pot on a similar discussion going on, on a BMW enthusiasts forum right now?
One of the themes on this particular discussion is the effect of increasing speed on economy and how it affects different engine and gearbox combinations differently.
Big engines see a much slower increase in fuel used as speed increases.

[tomstickland]

Well, yes, fuel consumtion is proportional to;
1) power requirement
multiplied by
2) bsfc (brake specific fuel consumption) at that speed/load combination.

So, you would expect an increase in speed to cause an increase in fuel consumption, but the rate of increase would depend on the engine bsfc performance. Small engines have to be thrashed to put out high power, whereas large engines don't. It's all about running the engine near its optimum power output.

So a 1.1 Petrol engine might be most efficient making 30BHP, so a steady 65mph cruise is its optimum bsfc point and the power requirement is quite low, so overall mpg is good.

A 3.0 engine might be at optimum bsfc at 80BHP, which would require a speed of about 90mph. So it would be running at its best bsfc, but the power requirement would be higher, hence worse mpg overall.

[loofer]

There's no science to back this up but here's my theory anyway.

I think most efficienct for 'my car' is in top gear (6th) where the torque band is at it's peak.
For my VW Bora TDI this starts from about 1950rpm I think.

Thankfully for me - 2000rpm is 70mph...so observing that speed limit is most efficient contsant speed - me thinks.

Diesel car's have alot more torque than their petrol equivalants and this peak torque is achieved much lower down the rev range.

[AdrianHi]

There's no science to back this up but here's my theory anyway.

I think most efficienct for 'my car' is in top gear (6th) where the torque band is at it's peak.
For my VW Bora TDI this starts from about 1950rpm I think.

Thankfully for me - 2000rpm is 70mph...so observing that speed limit is most efficient contsant speed - me thinks.

Diesel car's have alot more torque than their petrol equivalants and this peak torque is achieved much lower down the rev range.
Does quite work like that.
The TDi engines are extremely good, and after seeing the following graph for the first time, I see it as one of the best all rounders for fuel economy. The graph is for a Gold GTi TDi, so your's may a different profile - expect consumption to get worse quicker as speed climbs, but you get an idea.

http://www.metrompg.com/zoom_image.htm?H=437&W=640&ZoomFile=gcc%2Dautobild1%2Egif&Caption=%28%3Ca%20href%3D%22http%3A%2F%2Fwww%2Egre encarcongress%2Ecom%2F2006%2F05%2Ffuel%5Fconsumpti o%2Ehtml%22%20target%3D%22%5Fblank%22%3ESource%3C% 2Fa%3E%29&title=GCC%20AutoBild%20graph#

[tomstickland]

There's no rule that peak torque should correspond to peak efficiency. It's generally more rpm orientated, and somewhere around 2000-2500rpm.
A bit more cam and an engine might make peak torque at 5000rpm, but peak efficiency would still be at lower rpm.

[bryanb]

Anybody know a way for a layman to determine the most economic (sensible) speed for a given vehicle. Particularly a 1.9 non turbo, non common rail, diesel, without a fuel computer fitted? I realise I could conduct tiresome experiments driving at constant speeds etc but thing don't work like that.

[AdrianHi]

Anybody know a way for a layman to determine the most economic (sensible) speed for a given vehicle. Particularly a 1.9 non turbo, non common rail, diesel, without a fuel computer fitted? I realise I could conduct tiresome experiments driving at constant speeds etc but thing don't work like that.
Unless you going to get into varying speed and using momentum down hills etc. to shave the last bit off your fuel use, I think it's best to go with the flow of traffic, or if your own your own, what ever minimum speed you need to do to hang on to top gear. The actual speed may depend on the nature of the road. If you have a road going up and down hills a bit you might need to pick a higher speed to hang onto that gear up hill.
There is an interesting article on What Car which I would post a link to, but I keep finding links I make to What Car web site getting delete by mods, so have a search for "Fuel economy: making every gallon count - The test findings" article 6 June 2008.

[tomstickland]

I was reading What Car whilst waiting at the dentist. The Aug 08 issue has a report on mpg, including detailed measurements. All of the vehicles tested produced better mpg as speeds were reduced, with the best results at the lowest speed that they tested at, which was 40mph.

[tomstickland]

If that's so, what is the point of streamlining? Is it perhaps less turbulent if streamlined?
Having turbulent flow doesn't make streamlining redundant. The profile of the vehicle will still affect the aerodynamic drag forces.

[AdrianHi]

I was reading What Car whilst waiting at the dentist. The Aug 08 issue has a report on mpg, including detailed measurements. All of the vehicles tested produced better mpg as speeds were reduced, with the best results at the lowest speed that they tested at, which was 40mph.

My car has a 6th gear which I cannot use below about 50mph, however all 5 speed manual cars I've had would do 40mph in 5th so I would expect it to be the most economical speed on a reasonably flat road for these.

[andor]

Always got about 330 miles on a full tank of petrol until we decided not to exceed 60. Now we get somewhere in the region of 370 to 380 miles on a full tank.

PS. Apart from on holidays where we had to drive on about 400 miles of motorway.