In order to determine the best way to simulate a road in the full-scale climatic wind tunnel of the Voklswagenwerk AG, a series of comparative tests between road and tunnel have been carried out. In general, a good correlation was achieved when a rigid ground plane was used for road simulation. Only a small margin for improvement by boundary-layer control remains. From measurements reported in the literature, it can be seen that an effective reduction of the boundary-layer displacement thickness of 60% can be achieved with single-slot suction. Nevertheless, this would result in a hardly noticeable improvement of test-data correlation for cars with standard ground clearance. Therefore, it was decided not to install a boundary-layer suction in the wind tunnel of Volkswagenwerk AG.