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I started out with wheel covers and then later installed wheel skirts, leaving the wheel covers in place for over a year under the skirts. I got a mileage boost with the covers, another (larger) increase with the skirts, but no noticable mileage drop when I removed the covers. Once the skirts were in place, the top of the wheel was no longer exposed to the up to twice ground speed air velocity alongside the car, but rather is exposed to a squirrel cage-like rotational airflow inside the wheel well powered by the rotating wheel, so the wheel cover had little effect on total drag from inside the skirted wheel well.
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