agreed, the air resistance increases as the cube of the car speed so doubling the speed will produce an increase of eight times in the air resistance, without even considering rolling resistance which also increases with increasing speed. So why isn't the most economical speed 1mph then ?? consider the extreme case - standing still and engine idling, fuel is still being used , the friction in the engine has to be overcome, fuel pumps, alternator etc are still being driven. So like many technological/engineering solutions we end up in a compromise - balance whilst I don't know if there is some fundamental reason that cars tend to be most economical at 56mph the fact that this is the figure that manufacturers have to quote comsumption figures leads them to select gearing etc to make this 'headline' figure as high as possible. ps Mattjr - Its a bit like the maximum power theorem - most power is transferred when internal and external resistances are equal in a circuit (see also impedance matching) these are curves with a maxima.