The open jet wind tunnel is a widespread test section configuration for developing full scale passenger cars in the automotive industry. However, using a realizable nozzle cross section for cost effective aerodynamic development is always connected to the presence of wind tunnel effects. Wind tunnel wall interferences which are not present under open road conditions, can affect the measurement of aerodynamic forces. Thus, wind tunnel corrections may be required. This work contains the results of a CFD (Computational Fluid Dynamics) approach using unsteady Delayed Detached Eddy Simulations (DDES) to evaluate wind tunnel interferences for open jet test sections. The Full Scale DrivAer reference geometry of the Technical University of Munich (TUM) using different rear end shapes has been selected for these investigations. Experimental results from measurements of the 40% scaled wind tunnel model confirm the relevance of wind tunnel effects for different blockage ratios and test section characteristics at the wind tunnels of the TUM and Audi. The DrivAer geometry has been applied to CFD to simulate open road conditions in order to validate the aerodynamic forces with experimental data as well as set a reference for wind tunnel simulations. The simulation of the open jet test section including the DrivAer model has been applied to wind tunnels with nozzle cross sections between 11.0 m2 and 89.6 m2. The results are then compared to potential flow models by Wickern as well as Mercker and Cooper. Finally, conclusions are derived for the selected geometries in test sections with the plenum and the nozzle method.