A little (lot) of polygon overkill but the end result is looking alright. Flat headlights and tail lights is a bit 2002 GTA3 modding stylee but I expect your next car will be more 3D. You could just do a simple R* style engine and then start working on a new, polygon-efficient car?
As for the wheels, the reason that any wheels larger than milk bottle tops look weird is because the ground cleared (suspension lower limit) is too small. If it is currently -0.150 make it -0.200 and if it is something different then change it by -0.050 and see how that looks. You will need to recalculate the suspension force and effect after making this change. (
Rescaler.)