In response to growing concern over the validity of the EPA fuel economy numbers, the Department of Energy undertook a study of actual on-road fuel economy as it compares to the EPA numbers. This report covers the development of the data base for that study, the analysis techniques used for the initial phases of the work, and the preliminary results of that analysis.Data on over 5000 in-use vehicles were collected for model years 1974-1977. Data were obtained from a number of private and government groups for vehicles in fleet and typical consumer use, from on-road tests, and from in-use dynamometer tests. Comparisons using linear regressions were made between these mpg values and the EPA certification results for the same models.The results describe these differences as a function of vehicle mpg and model year. Other more specific comparisons are also made. An analysis of in-use fuel economy ranking, compared to the ranking by the EPA mpg numbers, is presented.