Report the drag force: how does that look?
I'd question k-e for this type of work, along with near wall mesh resolution but that shouldn't give too great an error. Have a look at the airfoil/aerofoil example/tutorial for setting guidance.
For the domain size. Run the model and look at both pressure & velocity contours and velocity vectors. How sensible is the flow, and how does the "sky" interact with the car(s)?
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