Just a thought about your wheels. I'm doing more or less the same thing: a 30% scale model for a "roof rack wind tunnel".
Automotive body engineering - Roof Rack "Wind Tunnel": A Naive Idea?
McMaster-Carr sells fairly cheap 8" polypropylene wheels with bearings, if you want (#2781T48), so you could do rotating wheel simulation. That's what I'm planning to do. 8" @ 30% scales pretty nicely.
I hope to pay a lot of attention to the wheel-groundplane intersection, particularly with those nasty trailng vortices.