The EPA urban and highway fuel economies were determined for six 1970 cars and 22 each 1974 and 1975 cars. These experimental data were combined with data from the literature to determine the effect on fuel economy of meeting past and current exhaust emission standards and to project the effect of meeting various levels of standards in the future. An average loss in fuel economy of 14% took place from 1967 to 1974 but about one-half of the loss was recovered in 1975. The requirement to meet more stringent standards in the future is projected to cause significant additional losses in fuel economy. Adoption of the emission standards recently proposed by the EPA could delay for several years the achievement of the 40% improvement in fuel economy requested of the automotive industry by the government. Imposition of the statutory standards mandated by the Clean Air Act apparently would prevent the achievement of the 40% improvement goal.In a companion program the improvement in fuel economy as a function of the increase in octane requirement was determined for a 34-car fleet of 1970 to 1975 model cars. This information was combined with estimates by the petroleum industry of the additional energy required to increase the pool octane number to determine the optimum octane number of unleaded and leaded gasoline to minimize the consumption of crude oil. Use of the optimum octane quality unleaded gasoline of 96 Research would reduce crude usage by 2% compared with current unleaded gasoline of 91 octane number. However, an investment of up to $20 billion would be required by the petroleum industry to increase unleaded pool octane number from current to optimum levels. Similarly, the use of the optimum octane quality leaded gasoline of 100 Research would save 2% crude oil compared with the current leaded gasolines of 95 pool octane number. However, the investment required would be much less than for unleaded gasoline. The use of leaded rather than unleaded gasoline, whether at current or optimum levels, will save 6 to 10% of the crude oil required.