If cars were more fuel efficient, we would get more from the same amount of fuel. heat useable energy gasoline

The answer is; usable energy

Efficiency is basically doing work with little wastage. Fuel is burned in engines to provide rotary motion. However, most of this energy from the fuel is lost as heat and only a small amount is converted to motion.  Increasing the amount of energy from the fuel that is converted to motion (concomitantly reducing waste energy) is basically increasing efficiency.