I have read two schools of thiught on th subject. (1) The Deflector adds drag and not mulch else but thr window is clean, (2) That the deflector does add to drag but in a very small way, it dous help with the bug but it also moves the airpresser higher up on the windshild. So that presser only has to be against that flat windshiled halve as long before it goes over the cab of the truck therefor redusing the overall dragg,
PhileasFogg 
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