[teoman]

Just to ponder on the question a bit further...

The unknowns were friction and air drag, these increase with speed, as you are below the set speed these will always be lower than stopped or accelerating.

Lets still assume that the guy was traveling at 40 mph.
And lets assume that he consumes 4l/100km @ 40 mph.

All of the energy in the fuel is going to combat air drag and other frictions. As the other dude mentioned if we were in space, once you accelerated to 40mph you would continue untill infinity.


Lets assume that you stopped in 200 m and re-accelerated in 300m. So total distance is 500m.

4000ml / (100.000) = 0.04 ml of fuel per meter.

We pulled 500m from a well calibrated deep dark datasource...
500m * 0.04ml/m = 20ml of fuel consumed in 500m.

If we assume that the speed was linearly decreased and linearly increased and drag and friction are linear with speed (which they are absolutely NOT, but for a slow speed of 40mph we can assume that it is without introducing an error of several orders of magnitude).

Then the total drag and friction is exactly half of that value... so 10ml of fuel.

Obviously you meed to plug in the appropriate numbers. So you wasted a total of 11ml + 10ml of fuel for the stop and go.

Had you not slowed down at all, you would have used only 20ml of fuel.

Bu you did travel at 20mph for 500m of the route and lost time.

Obviously you need proper data sources for these calculations.


(And had you traveled at 20mph all along, you would have consumed 10ml of fuel in total, so you wasted 11 ml from a general efficiency perspective, apples to apples comparison where total distance and time are equal).