Not necessarily a real wind tunnel, but I seem to remember someone attaching 100's of strings all over their car and driving at various speeds with a chase car video taping the the resultant airflow patterns (hell, maybe I dreamed it ). I believe it was on the PP board a few years back. If someone had some extra time on their hands and a buddy with a video camera, I for one would be very interested to see exactly what goes on at various speeds (including "high speed" runs), and different views (front, side, rear), and even "top on" and "topless".
Just a thought.