Nevyn -- As shown above, that was my thinking. I didn't mention the fans forcing air like the Chapparal race-cars:
Although if little air gets underneath in the first place, the air flowing over the open back window might suck (like an airbrush) enough to work passively.
In the 1920's R. B. Fuller proposed the opposite with his Omin-directional Transport:
(Hey, 'pneumatic guard rail'——airbags in the 1920s) But of course he wanted to fly the rear wheel with a horizontal elevator. The only other person I heard of doing that was 'Jocko' Johnson:
A passing mention after some interested discussion of transonic/supersonic cars here:
July 2009 Update
And of course Bucky dropped it in his Dymaxion car.
I'm hoping MTrenk drops into the thread.