nabasa ko rin somewhere that each car uses only approximately 15% of its total energy output to turn the wheels. dami kasing moving parts and other variables kaya daming energy lost sa isang auto due to friction and heat.

On another topic, nabasa ko rin pala na ang drag is exponential. forgot the exact formula but its something like <some variable> x velocity squared. So ibig sabihin the drag that a vehicle encounters at 80kph is four times the drag at 40kph. I guess to achieve maximum fuel efficiency, one must balance it between drag and travel time (since the car travelling at 80kph experiences 4x as much drag but for only half the time as a car travelling at 40kph for the same distance).