Many aerodynamic wind tunnels used for testing of ground vehicles have advanced ground simulation systems to account for the relative motion between the ground and the vehicle. One commonly used approach for ground simulation is a five belt system, where moving belts are used, often in conjunction with distributed suction and tangential blowing that reduces the displacement thickness of the boundary layer along the wind tunnel floor. This paper investigates the effects from aft-belt tangential blowing in the Volvo Cars Aerodynamic wind tunnel. First the uniformity of the boundary layer thickness downstream of the blowing slots is examined in the empty tunnel. This is followed by investigations of how the measured performance of different vehicle types in several configurations, typically tested in routine aerodynamic development work, depends on whether the tangential blowing system is active or not. Numerical simulations are also used to explain the flow field origin of the force differences measured in the wind tunnel. Results show that even though the displacement thickness behind the blowers varies along the width of the blowing slots, it is significantly reduced compared to the case of no blowing; furthermore, it is also shown that deactivating the blowing altogether has an effect not only on the absolute forces but also on the deltas measured between different configurations, and that this phenomenon is more prominent if the vehicle has a larger base area.