With my truck, with 3.55s, 305 Coopers, I was getting around 16.5 easily -maybe 18 mpg on good day stock. Since BD came out with the tunes for the 12 models late last year, it started getting a little better mileage. With S&B, Bully Dog, Mufflex, 180 TS, catch can, it now can easily get at least 18 on a bad windy day (hand calculated) and I have seen up to 20 several times. When hand calculating my evic is usually about a half mpg off of the hand calculation. I actually saw 22 mpg for 20 miles coming home the other night. I do NOT believe that. lol. I might have had a good tail wind. But I have never changed my driving style at all, and the truck was already well broken in before I started modding it. So, I actually believe you can see a mileage improvement with mods like these because I have seen 38,000 miles of driving with probably 75 percent of those miles being highway travel with the cruise on. I run premium tune with 93 octane all the time. I even had a bug shield on it for 35,000 of those miles. Go ahead and mod your truck. When you need warranty work done, have your dealer do some work like tranny fluid change or oil change or such at the same time. If you are spending money there on occasion I'd bet they will be a LOT more forgiving of flashes. Also I don't mean to be too critical of the flowmaster (I LOVE the sound of them) but I think a Magnaflow or another straight thru type muffler will net you more flow and performance / mpg. Plus they don't rot out like the FMs do. Enjoy your modding