Car design factors which affect fuel economy include body size and shape, vehicle weight, engine size and power train design. Computer simulation results are presented to illustrate some of the basic interactions which occur among these parameters. Exhaust emission standards and other practical constraints limit the quantitative usefulness of such parameter variation studies. The fundamental relationships between engine efficiency and exhaust emissions are reviewed. It is concluded that emission constraints are important and can limit the effectiveness of some design changes to improve fuel economy. With present technology, it can be shown that stringent exhaust emission standards will reduce fuel economy more than it can be improved by car design changes which do not seriously degrade many customer perceived values such as useful capacity, driveability and acceleration capability.