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I have used the covers and skirts together. My testing did not compare skirts to covers, I just did each on there own and all my mods together.
But my wheel skirts gave me .4-.8 MPG (best estimate of .5-.6).
My wheel covers gave my 1.6 MPG.
So if I look at the MPG gain of just the back two wheels the covers should =.8MPG. This is more than the wheel skirts give me.
My thought is that the wheel covers also keep air from being pushed out (widening the car- think mist from semi truck on wet day) which is why they gain me more MPG.
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