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The smooth wheel covers generally get better mileage because the wheel itself has less drag, since a spoked or unsmooth wheel acts as a crude turbine. So, it takes less energy to turn a smooth wheel in the air than a rough one, all else equal, which it seldom is.
Clearly, from the videos above, that's not the whole story, as tufts in video 2 show more turbulence along the side of the car, especially in the immediate wake of the wheel well.
Another factor that may be at work here is that since many wheels act as turbines and throw columns of air out sideways, they increase the effective frontal area of the car. We saw this clearly when a group of Porsche 944s went ratracing at high speed. Driving along wet roads, it was obvious that some wheels (on otherwise identical cars) threw out lateral columns of air 2-4' sideways, whereas smooth wheels like my D90 or the Porsche "sewer lid" types generated much less side flow, for less effective frontal area. Could be in the videos above, there is more turbulence in #2 because the sideflow hugs the car better, whereas #1 does not. It would be interesting to have tufts on sticks further out in the slipstream, to see what's happening to air flow 6", 12", 24" etc. out from the surface.
Also, how about repeat this test with tufts taped to the wheels themselves?
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