The optimization on an engine-three way catalyst-vehicle system for best fuel economy under the constraints of emissions and driveability, has been studied for a smaller passenger car with 1.6 litter 4 cylinder engine. Full utilization of a computer was intended for the accumulation of basic data and a search for optimum calibration. An automatic Engine Test System ( AETS ) was developed. The optimum calibration was calculated for the engine-vehicle system with three-way catalyst based on the optimal control theory, and was implemented on an actual vehicle by an engine controller in which a 16 bits microprocessor was used. The results showed good agreement.