Cars have geared transmissions. Cars designed for the Autobahn (most high end German cars) have gearing optimized for decent cruise range at 120-180 km/h. The

Cars have geared transmissions. Cars designed for the Autobahn (most high end German cars) have gearing optimized for decent cruise range at 120-180 km/h. The engine is sized for peak acceleration (and marketing). It has a turbo (in the 535d; the 550d has 3....), which helps somewhat without adding more fuel consumption, but still, a 300hp engine is pretty big. Basically, to cruise along at 120kph in a reasonable car with reasonable Cd probably is about 20-30hp (yay for mixing metric and imperial units...). Wind is probably 20hp of that. The engine is less efficient at low output (specific output), so you won't get fuel consumption at 10% power which is only 10% of peak fuel consumption. Wind resistance does go up, but your engine also gets to operate more within its power band. With the right gearing, you can stay within the optimal RPMs and be close to the design power level of the engine at a reasonable cruise speed. Empirically, I've driven big heavy cars fast for long distances, and gotten reasonable fuel economy; especially big German cars with big diesel engines. What kills them is stop and go, or when you have a smaller car with, say, 5 gears, and you are running it at redline to keep your desired speed. What is awesome about electric cars is you could get much more efficiency in the motor across a range of power levels, plus regen. What is not at all awesome is the power density of lithium ion batteries.