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With the help of theoretical considerations it is shown that the flow between a car and the ground is of boundary layer type, as long as there is no recirculation. Thus, boundary layer theory can be applied to evaluate the order of magnitude of typical effects like displacement and momentum-loss thicknesses of ground-plane boundary layer. If the boundary layer in a wind tunnel with stationary ground is to be controlled, either by distributed suction or by tangential blowing, to simulate on-road conditions, boundary layer theory can provide the orders of magnitude of modifications that have to be applied. Experiments with a ground-effect quarter-scale car with ground simulation by distributed suction and moving belt showed coincidence with theoretical predictions concerning the required suction rate, if integral coefficients (eg CD, CL) of both flow cases are matched.