Both the external and internal flows of cars are simulated simultaneously. A third-order upwind-difference scheme is used in these simulations. Computational grids are generated by a multi-block transformation and a trans-finite method. Engine compartments are modeled by grid systems but the heat exchanger is simulated as a pressure loss proportional to the dynamic pressure of the flow passing through it.First, the flow for a very simple test model with no wheels and nothing in its engine compartment is simulated and compared with experimental results in order to validate a simulation method for the engine compartment. Pressure distributions on the inner surfaces agree very well with measured values, while pressure distributions on the external surfaces show reasonable agreement except for the roof end and the leading edge of the floor. The predicted drag coefficient is 7% larger than the experimental value.This method is next applied to a prototype car. A grid system is generated including an engine block, a transmission, an air-cleaner and suspension arms. Both the drag coefficient and the airflow rate through the radiator are calculated. These values are compared with experimental results and the agreement is good. We conclude that the present method is quite useful for predicting and optimizing both the drag coefficient and the airflow rate through the radiator.