Those are both good points, it would make more air go over the top of the car rather than under, but I could see how maybe an under tray would be more effective at adding mpg. I will say that I think the rear diffuser would be better for mpg because it helps smooth out the air coming off the back of the car. It's like vortex generators for under the back of the car! I think there could be something there.
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Best fill up so far is now 29 MPG!
collegekid01 
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