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Try leaving the Drews Brew alone for a few tanks. My truck didn't seem to like it. Also if you have your stocker tires put them back on and then check your mileage. I have also taken off my XZT and am running stock except for the different (MBRP) muffler.
People get mad at me cause it takes me 15+ minutes to fill my tank. I slow squeez in 5 gallons after the pump kicks off. It takes for ever to get all the foam down and fill it to the very top of the filler neck until it doesn't go down any more.