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If you are driving 70+ MPH, then engine accessory losses are small compared to aerodynamic drag. Your truck is already geared for mileage. If there's almost no load on the engine, a taller rear end ratio will help somewhat.
Go after the big aerodynamic losses - grille block, air dam, belly pan, and aero topper. I got more benefit from the grille block than my aero topper. That's also the easiest mod.
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