Help with a race car CFD analysis

Hello,

Recently I came across an accurate modle of a Porsche 944 and I desided to do a CFD analysis in order to calculate the Cd and Cl. Now I ran the simulation and got a 45% error on ther Cd (0.33 Claimed, 0.5 Measured). I relize that it wont be perfect because the model isn't 100% accurate, however, I would happy with at least a 10% error.

During the simulation i set the initial air velocity to 160 km/h however, when the simulation finished the air velocity was 136 km/h. I dont understant how this happens because I set the velocity as a constant.

From what I have read so far the mesh is KEY to have accurate results. So basically I need help setting up my mesh.

Mesh Info:

RefinementLevel
Curvature Refinement Level4
Tolerence Refinement Level3
Refine Fluid Cells2
Refine Partial Cells4
Refine All Cells-
Narrow Channels Refinement Level1
Initial Mesh Level7

If you need any more information just let me. I can also provide the actul CAD model and pictures if needed.

I would greatly appreciate any information you guys can provide and thanks in advance.

Thanks,

Mark

SolidworksFlow Simulation