Fuel Consumption - high speed vs low speed

[stoory]

Why is it that if you drive from a - b at a high speed then you use more fuel that driving a - b at a lower speed?

I'm aware that at a high speed the engine obviously works harder and therefore consumes more fuel... but surely if you go slower, although the engine doesn't have to work as hard, consuming less fuel, it has to do it for a longer period of time. Therefore you may think that it would even out and be the same... but it doesn't?

So, my question is... why don't they even out?? I believe its something to do with the engine being less effective at high speed but would love to hear a clearer explanation ... in laymans terms please!!

[Tim Baxter]

Simple answer, air resistance.  The faster you go the more air you have to shift so the more power you need so the more fuel you burn.

The most efficient speed is the slowest that your highest gear will handle.  For example, my 2 litre turbo diesel will do 65mpg at 40mph, 60mpg at 50mph, 50mpg at 60mph etc.  It won't handle any speed on the flat in 5th less than 40mph so to slow down and change to 4th would increase the revs and consequently the consumption.

Hope this helps

[hedgehog661]

Forget the time, you are still covering the same distance (work done) at the faster speed the engine is using more power = more fuel,

If you did the distance on a push bike you would still cover the same distance but your fuel would be the food you eat (might take a long time though

work = force x distance moved