My mpg got a lot worse with a Superchip tuner and AFE CAI, they make my truck touchy on the gas pedal causing me to use more gas to get my truck moving, I also carry about 500lbs a day in it for work. So I went from the 15-18mpg range to the 12-16mpg range. But like I once heard "friends don't let friends drive stock"......
That's strange....wonder if you were just giving it more pedal enjoying the more power? I definitely get at a minimum 1mpg better with my superchips on 91 or 93 tune. I'd say more like 2 and MAYBE even 3 better, but 3 might be a stretch. I was running the 91 tune because I was getting some pinging with 93 and a load, but went back to the 93 tune and as long as I don't have any weight in the truck it's fine....not really a noticeable difference over the 91 tune though.