Fuel economy data on 1975–1978 California production automobiles was generated by three methods; the approved carbon balance method, a direct/gravimetric method, and in-use fuel economy. Both low mileage (0–15,000 mile) and high mileage (40,000–60,000 mile) data were acquired. The data was analyzed on a paired automobile basis, and also by engine family, to determine percent differences between: -- EPA estimates and production automobile fuel economy. -- the fuel economy measurement methods. -- the fuel economy of automobiles at low and high mileage.The results and conclusions indicate significant differences between EPA estimates and production automobile fuel economy between fuel economy measurement methods, and fuel economy changes due to mileage accumulation.