Reducing resistance forces all over the vehicle is the most sustainable way to reduce fuel consumption. Aerodynamic drag is the dominating resistance force at highway speeds, and the power required to overcome this force increases by the power three of speed. The exterior body and especially the under-body and rear-end geometry of a passenger car are significant contributors to the overall aerodynamic drag. To reduce the aerodynamic drag it is of great importance to have a good pressure recovery at the rear. Since pressure drag is the dominating aerodynamic drag force for a passenger vehicle, the drag force will be a measure of the difference between the pressure in front and at the rear. There is high stagnation pressure at the front which requires a base pressure as high as possible. The pressure will recover from the sides by a taper angle, from the top by the rear wind screen, and from the bottom, by a diffuser. It is not necessarily the case that an optimized lower part of the rear end for a wagon-type car has the same performance as for a sedan or hatch-back car. This study focused on the function of an under-body diffuser applied to a sedan and wagon car. The diffuser geometry was chosen from a feasibility stand-point of a production vehicle such as a passenger car. The fluid dynamic function and theory of the automotive under-body diffuser working as a drag reduction device is discussed. The flow physics of the under-body and the wake was analyzed to understand the diffuser behaviour in its application to lift and drag forces on a vehicle in ground proximity. This work is mainly a numerical analysis that uses the traditional CFD approach from the automotive industry. Results from this study show a potential to reduce aerodynamic drag of the sedan car approximately 10%, and the wagon car by 2-3 %. The possible gain was much bigger for the sedan vehicle and the optimum occurs at a higher diffuser angle. This was most likely due to the fact that the sedan car in its original shape produced more lift force than the wagon, a wagon usually produces very little lift or even down-force. Lift forces were also reduced with the use of under-body covers with diffuser. The down-force increased, or lift force decreased, linearly with increased diffuser angle, and the trend was the same for both sedan and wagon rear ends. Flow analysis of the wake showed the importance of how the wake is balanced.