For the wind tunnel testing of race car aerodynamics, it is essential to cover the real underbody flow field characteristics, as they affect the aerodynamic balance of race cars and, hence, the transferability of wind tunnel measurements to on-road conditions. In this study, numerical simulations have been performed to determine the influence of moving belt dimensions on the aerodynamic forces for a particular race car. The results are compared with an infinitely large moving ground in open road conditions. Thus, the requirements for the dimensioning of a single belt moving ground simulation in wind tunnels for race car aerodynamics are derived.