many studies have been done, both by manufacturers and independent testers, there is nothing that makes any difference as far as tailgates or bed covers, the pick up design has been wind tunnel tested, there is a natural low pressure zone the air flows over the cab, a low pressure vortices's builds up and the air flows over the tailgate, all that tailgate down tailgate up, no tailgate, backwards hinges, air gate none of it is worth bulldookey. best you can do is as you crest a hill lift your foot slightly and let momentum carry you down, this can actually work pretty good a good tune up clean air oil and pcv filters and clean fuel filter and a good brand of gas, with a light foot is about the best it ever gets.