Ansys has recently published a free course on simulating the external aerodynamic analysis of a car. Please find the link below. You can find the right guidance related to meshing and solver setup from it. It is likely that the boundary layer is not resolved properly in your case and that is giving an error in the Cd value. Also, Spallart Almaras is not the best choice of turbulence model for this case. For details on tubulence modeling, please refer the course mentioned below.