This paper compares the fuel consumption of a lightweight passenger car for three different SI engine concepts, all with rated power of about 40 kW: a classical SI engine with moderate maximum speed, a low-displacement but high-speed engine that exploits the maximum allowed mean-piston speed and a low-displacement but highly supercharged engine with moderate maximum speed. All engines are simulated with a thermodynamic process simulator, the results of the supercharged version are validated with experiments. For each engine, a CVT and an automated gearbox is considered. Fuel consumption is estimated with a quasi-static driving cycle simulator which is based on engine fuel consumption maps and physical models of the vehicle with all its relevant subsystems. The simulations are performed for constant vehicle speed as well as for US and European driving cycles. The results show the advantages and disadvantages of the different engine and transmission concepts regarding their efficiency in vehicle use.