Driving Economically - 56mph Myth??

Murphy_The_Cat

jdm01 wrote:

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

MTC

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

tomstickland wrote:

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

Idiophreak wrote:

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

tomstickland wrote:

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

MarkyMarkD wrote:

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

wolvoman wrote:

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.