Driving Economically - 56mph Myth??

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

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

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Bunking_off, you are out of date.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

Murphy_The_Cat wrote:

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

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

Cheers...J

Wig

jellycat40 wrote:

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

tomstickland wrote:

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

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.