Many factors can be cited which produce differences between the fuel economy values obtained during the exhaust emission certification process and the economy experienced by car owners. Admittedly, all laboratory tests are compromised by many assumptions, approximations, and practical test limitations. The main value of the EPA test procedure is that it has provided a uniform test method for all manufacturers which produces vast amounts of comparative fuel economy information. Changes to the procedure to make it more “representative” have reduced its usefulness for comparisons to previous years.The concept of labeling cars with a “representative” fuel economy value is certain to result in some customer misinformation and dissatisfaction. At best, current labeling methods can be expected to indicate real vehicle differences only when label values differ by more than 2 mi/gal (0.85 km/l). Furthermore, wide variations in customer fuel economy (ranging up to 15 mi/gal) for the same EPA label value are bound to make some people regard the label values as misleading.Changes in new car fuel economy have a significant impact upon future fuel demand projections. A stable fuel economy measurement procedure and an understanding of the factors which relate certification to average customer economy are needed to reduce the uncertainties in such a projection.