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I saw similar differences in outflow from various wheel designs when about 30 of us with Porsche 944 cars did a 500 mile ratrace through British Columbia. This was a long line of otherwise identical cars over the same wet roadways at the same speed: Open-spoke car wheels threw out a lateral column of misty air several feet into the slipstream, whereas cars with smoother, more closed shapes more like pizza pan covers did not throw out such lateral mist into the slipstream. I believe the latter type wheels mean effectively less frontal area for those cars, less disruption of the slipstream, and surely better fuel economy. Smooth wheel covers would also make less drag than spokes, which also may act as crude turbines.
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