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That roof rack setup on the cap is really going to hurt your mileage. Otherwise, you would probably see a slight gain with the cap. A tonneau cover would be better for aero, though, with a custom aero cap being better still. My dad has always driven 3/4-ton trucks, and he's found that his current truck (2010 F-150, 4x4, loaded) gets no significant FE change from a full camper shell. He did, however, see an improvement on both his '03 F-150 (FX4 and well-appointed) and '94 Chevy 1500 (350 V8, auto, 2WD). I don't remember talking to him about how the mileage was on his '85 ranger before that.
My family has always calculated our mileage when we fill up, and we have noticed a marginal improvement in FE across the board with the above trucks (never had it on the Ranger) as well as a few Subarus, a Corvette, a Mustang, and a Maxima, with the K&N drop-in elements. Perhaps it doesn't show results in some easily-testable situations, but we've always seen 1-2 MPG improvement on a tank-to-tank basis.
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