A review is made of available data on fuel economies of the current internal combustion engine-powered automobiles and of those with alternative powerplants. Comparisons of fuel economies of all these engine systems are made on the basis of the vehicle weight/engine displacement, and the vehicle weight alone. The thermal efficiencies are also compared. It is shown that several versions of the diesel engine which meet the 1975 Clean Air Act Standards and which are on the road today are more efficient than the conventional internal combustion engine of 1973. Moreover, prototypes of other alternative systems, using other cycles (Brayton, Rankine, Stirling) under development are also projected to provide higher efficiencies than the conventional internal combustion engine of 1973. All comparisons are made using the federal driving cycle as a common reference.