To investigate the fuel-economy benefits of increased compression ratio in modern emission-controlled engines, the compression ratios of two car makes were increased one unit while holding emission levels and performance nearly constant. All cars were modified for lean-burn operation by using the Turbulent Flow Manifold, a carburetor adjusted for lean air-fuel mixtures, and special calibration of ignition timing and exhaust gas recycle rate. Exhaust port liners and a deceleration valve were used to control the tendency for HC emissions to increase at higher compression ratios.In addition, thermal reactors were installed on one high-compression car. The increased efficiency of this aftertreatment device allowed a simultaneous improvement in fuel economy and decrease in hydrocarbon emissions. Based on fuel economy gains and refining savings, this car offers an 8.2% fuel savings over its 1977 certification counterpart.