This paper discusses some trends and influencing factors in passenger car fuel economy. Fuel economy and fuel consumption were calculated by a carbon balance method from HC, CO, and CO2 emissions measured by the 1972 Federal Test Procedure.The information presented was derived from nearly 4000 tests of passenger cars ranging from 1957 production models to 1975 prototypes. Data are presented for various model year and vehicle weight categories.Trends in fuel economy are discussed on an overall sales-weighted basis and for each individual weight class. Some of the factors that influence fuel economy are quantified through the use of a regression analysis. Particular emphasis is placed on the differences in fuel economy between those vehicles that were subject to federal emission regulations and those vehicles that were not.Three ways to characterize vehicle specific fuel consumption are presented and discussed. Possible ways to improve fuel economy and vehicle specific fuel consumption are also discussed.