[TheLaughingMan]

I just wanted to throw another thought in there to explain perhaps why the tufting looks more disturbed in the second (smooth wheel) video. The discs are covering the holes in the wheels, which would prevent any "venting" through the wheels. I think that the higher airspeed along the sides of the car (think bernouli effect of higher airspeed resulting in lower pressure) and the "compression" of air that passenger vehicles see underneath them would cause an outward movement of air through the spaces of the rotating wheel. I don't think that unless the wheel pattern is specifically designed to move air will there be any "fan" effect in either direction.

Therefore the wheel covers would prevent this outward flow of air. In the first video this outward flow is pushing turbulence away from the sides of the car, but when you reduce the amount of this flow the turbulence is closer to the sides of the vehicle and can be seen in the tuft testing.

As another post noted, there are many engineering papers that prove the worth of smooth hub caps. I think that we're only seeing a small piece of the whole picture which leads to improved efficiency.