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I think it is speed related. If your traveling faster the air would tend to go up the windshield and over the car, I think. At a slower speed it would tend to get into the rack more. Maybe you could build the upper half of an airfoil and mount it to the roof rack. Build it so the front matches the windshield angle and then becomes the "Template" from there to the rear, ending at the top of the tailgate.
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