If you were to design a car for optimal gas milage, slower is the way to go. There are two components of resistance that have to be overcome by power. The rolling resitance and the areodynamic drag. Power required to overcome rolling resistance is roughly proportional the speed of the car. For example if it takes 5 HP to go 50 mph it will take about 10 HP to go 100. Power required for areodynamic drag is proportional to the qube of speed. If it takes 5 HP to go 50 mph then it would take 125 to go 100 mph. Therfore a car with the above performance would require the sum of these powers

5HP rolling power + 5HP areodynamic power = 10 HP @ 50 mph
10HP rolling power + 125HP areodynamic power = 135 HP @ 100 mph.

10 HP is approximatly the required amount of power to drive the average car @ 50 mph on a level surface in no wind at sea level with standard atmosphere. So the above example is somewhat typical.

So from the cars point of view the power required is absolute minumum the slower you go. Now the engine and transmission put into todays cars are not very efficient at low speeds. This is because the engine is way off its power curve and the transmission's torque converter is slipping. Genreally a standard engine will have its best specific fuel consumtion SFC at peak torque RPM, at wide open throttle WOT, which is typically in the 70 mph range for most cars. The best SFC will occure at lower RPM's for off WOT conditions. The transmission will not lock up its torque converter under about 30 mph. so while the car is getting less efficient rapidly with speed, the drive train is getting more efficient.

I would estimate the optimum fuel economy for most US design cars occures in the 30 to 50 mph range for cars with automatic transmissions and 20 to 50 for cars with standards transmissions.

I would also say that German cars are more likely to get better milage at slightly higher speeds due to the design focus including driving on the Auto Bahn.

Stuff like variable valve timing and 7 speed transmisions allow the drive train to perform better over a wider range of conditions and therefore will result better gas milage at lower speeds.

John Roncallo