The fuel economy data compiled by the U.S. Environmental Protection Agency (EPA) have been analyzed to determine the trends in passenger car fuel economy beginning with model year 1958. Light duty truck fuel economy has been examined beginning with the 1976 model year. This paper adds the 1977 model year data to the historical trend and concentrates on the comparisons between the 1976 and 1977 models and the 1977 California and 49-state fleets.Calculation procedures have been used on the passenger car data which allow the changes in fuel economy due to system optimization, new engine/vehicle combinations, and weight mix shifts to be determined separately for individual manufacturers and the fleet as a whole. The passenger car fleet consists of the 13 manufacturers which were certified by September 13, 1976. Seven manufacturers comprise the light duty truck fleet.