[nmgolfer]

Twice as fast as the speed of the car? Speed of the car (wind) is Vcar=2*pi*r*omega where omega is the angular velocity i.e. rpm. Speed of the rolling surface of the tire relative to its center of rotation is Vrel= r*omega. So a point on the rolling surface of the tire is experience and alternating wind velocity: Vwind = Vcar +Vrel = (2*pi*r + rcos(theta))*omega depending on angle (theta) where it is in the rotation.

Remember there are two kinds of aerodynamic drag: skin friction and form (pressure). Ask yourself: what has more drag, two closely spaced plates moving in opposite direction or one plate moving at speed in a free-stream? When you cover the wheel well you're improving form drag no doubt.. but what have you done to the friction drag? There is now complex flow pattern between the wheel and the wheel well cover. I haven't run the numbers (or done testing) but I do wonder if skirts are worth it.

Most hub caps are like egg beaters chopping their way through the air. But they look good and they provide much needed cooling for brake pads. We can put smooth hub caps on a reduce both the noise (aeroacoustics) and the "egg beater" form drag... but we may need to replace the brake pads much sooner than otherwise (unless we drive a prius that is).

I'm just speculating but I believe if wheel well covers were such a huge benefit in drag reduction, we would be seeing more of them. Its not like the car companies aren't trying. But when average speeds are sub 50 mph they may not be worth it. Just saying.