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on the freeway the roofs seem to bubble or bulge out. It's because air in motion has lower pressure. More speed = lower pressure, its how an aircraft wing works too. The air inside the vert is not in motion so its pressure is greater than that outside the car's roof, so the roof bulges as the higher pressure air exerts itself against the lower pressure exterior air rushing by. Apply this principle to our wheels, inside the wheel space (where the brakes reside) there is a sort of unsealed pocket of air that is not in motion at the same rate as the air rushing past it outside the car. This air therefore has greater pressure than the air outside the wheel rushing by it (and also is heated by the brakes creating even more pressure) and will move towards exerting itself onto the outside air. The wheels can aid this pressure-driven "draw-through" effect by having a design that acts as a fan blade pulling air through them to the outside. Should your wheels be on backwards, they would be working counter-productively to what the laws of physics are doing with the natural airflow. The tension rod air guides simply deflect air into the interior pocket of air to even further enhance the whole process.
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