This paper describes the results of a study to evaluate the relationship of compression ratio on fuel energy conservation with the constraint of the 1977 Federal emission standards (1.5 HC, 15.0 CO and 2.0 NOx). The influence of the energy losses in the refinery process to produce higher octane fuels was considered as well as the effect of compression ratio on engine efficiency. Two different emission control systems were evaluated; a catalytic converter-EGR system and a manifold reactor-EGR system. These systems were evaluated on six vehicles; three intermediate size with 350 CID engines at compression ratios of 7.4, 8.3 and 9.2:1 and three sub-compact size with 151 CID engines at the same three compression ratios. Based upon total energy conservation, there does not appear to be an incentive for increasing unleaded or leaded fuel octane levels to allow for the use of higher compression ratios with converter-EGR or reactor-EGR control systems at the 1977 Federal emission standards. In addition, the catalytic converter-EGR system appears to provide equal to or improved energy utilization compared to the reactor-EGR control system.