I'm playing with some numbers... Assume: A car takes 15 hp to move at 65 mph, steady speed. 1. gasoline with 140,000 BTU/gallon engine efficiency is 20%, drivetrain is 93% In this case, the car should get 44 mpg. 2. fuel 150,000 BTU/gallon diesel engine at 40% efficiency The diesel car should get 95 mpg Which of my input numbers are way off? It seems that real diesel engines might only be operating at 20% efficiency, and gas engines at 15%.