Less than one-half of one percent of the energy in the fuel of a typical modern auto actually moves the driver. Only about 5% moves the auto. An auto's weight is responsible for more than two-thirds of the energy needed to move it. But each unit of energy saved at the wheels saves seven units at the tank by avoiding losses in converting and delivering it to the wheels, while a more efficient propulsion system has no such leverage. Lightweighting is the most powerful way to raise autos’ efficiency.
Sovran, G. et al. 2003. Contribution to Understanding Automotive Fuel Economy and Its Limits. Warrendale, PA: SAE International. link