The stationary ground-plane normally used in wind-tunnel testing generates a boundary layer in a model's approach flow which affects the measured aerodynamic characteristics. To establish the magnitude of this effect, the thickness of the ground-plane boundary layer relative to a model's size was varied. An automobile model of ″simplified″ shape and configuration in four different scales (1/8, ... [Show full abstract] 1/6, one quarter and 1/3) was used, and boundary layers of three different thicknesses were generated. Tests were conducted in three different wind tunnels. The relative boundary layer thickness was found to affect model drag and lift, particularly lift, to a sufficient degree that it should be accounted for in the wind-tunnel testing of ground vehicles.