A study was carried out that permitted determination of the impact of annual emissions inspection/maintenance on fuel economy. When 50% of a car population was rejected for high emissions and tuned up, the estimated gain in fuel economy among the rejected cars was 13%. This effect was calculated by quantifying the effects of individual engine malfunctions on fuel economy and applying the results statistically to a car population whose distribution of malfunctions was known. The approach used permitted calculation of fuel economy effects when distributions of some car characteristics, driving patterns, and engine malfunctions are known. The results apply only to pre-1975 cars. It was found that repairs carried out to correct only emissions should produce about three-quarters of the fuel economy benefit achieved by a complete tune-up. When deterioration between annual inspections was considered, and 50% of the car population was rejected for high emissions, the net gain in fuel economy among the rejected cars was about 7.5%, which was significant at the 95% confidence level. The net gain in fuel economy among rejected cars was not statistically significant at rejection rates lower than 30%. When the effect of repairing the 50% highest emitters was spread over the total car population, the estimated 2% fuel economy benefit was not significant.