The numerical and modeling errors in computational aerodynamics consist of several components. The errors are particularly large at the front in the stagnation region and in the rear in the base region. This paper presents results from numerical simulations of the flow around the Volvo ECC, see figure 1, and a generic vehicle shape. Several possible sources for the base pressure error are investigated. It was found that boundary conditions and mesh resolution affected the base pressure. Several turbulence models have been implemented and tested. The models range from a simple linear eddy viscosity model, EVM, to more complex models such as non-linear EVMs, explicit algebraic Reynolds stress models and Reynolds stress transport models.