http://www.windshearinc.com/images/40717_eprint.pdf for a one page pdf of the Road and Track Oct 2007 article... that goes into great detail to say... they figured out that putting a race car on a rolling track will add other real factors that a stationary wind tunnel can't reproduce. Think of a car on a really huge belt sander.
http://www.windshearinc.com/ Windshear is one of three full scale, single-belt, rolling road wind tunnels in the world and the only one available commercially.
Via
http://www.gizmag.com/go/8106/
