The fuel economy data obtained from the emission tests run by the U.S. Environmental Protection Agency (EPA) have been used to show passenger car fuel economy trends from model year 1957 to present. This paper adds the 1975 model year to the historical trend and concentrates on comparisons between the 1975 and 1974 models.
Methodologies which allow different 1975 vs 1974 comparisons to be made have been developed. These calculation procedures allow the changes in fuel economy to be determined separately for emission control systems, new engine-vehicle combinations and model mix shifts. Comparisons have been calculated not only for the fleet as a whole but for each of the 13 manufacturers who were certified as of the time this paper was prepared.
The net change in fuel economy for the fleet has been estimated at +13.8% comparing the 1975 models to the 1974 models assuming no model mix change occurs. The majority of this change, 11.5%, is attributable to emission control system refinements and engine optimization. General Motors is responsible for the majority of the fleet average improvement due to the combination of their large market fraction (approximately 40%) and their 28% improvement due to engine/control system optimization made possible through the use of oxidation catalyst technology.
The trend toward improved fuel economy which has been initiated by the 1975 models can continue in the future even with more stringent emission standards in effect, provided the achievement of good fuel economy remains a design constraint. Continuation of the trend may require the use of advanced hydrocarbon control systems which have not yet been fully developed.