MPG should not go down unless you start using the new found power then it will drop like a rock but then again the truck is accelerating like a rocket. I had a P-pumped cummins with a #10 fuel plate, AFC kit, 370 injectors, massaged Turbo, high flow exhaust man, straight pipped, bhaf, 191 delivery valves, timing advanced to 16.3 degrees, strapped to a NV4500 with SBC CONO Clutch and she turned 22mpg on a daily basis and that was with me at least 2-4 times a day rolling coal from a stop light just for funs and giggles. When I would get her out on the highway and NOT roll the smoke she did turn 24mpg (but that only happened a few times because I liked the power and the feeling of being held in my seat). But when I went and played with her hard I could run the mileage down to 8-10 mpg when showing off what she could do.... so by upgrading the power output you actually increase the mileage but YOU CAN"T USE THE POWER!