I called Banks and Gibson back when I bought my truck in January. Both said they do not have a cat back system that will fit. No idea when they would have one either, told me to call back in a few months. I did call back and emailed Banks and found out they finally made one to fit the 2011 2500HD 6.0 Vortec. Since the frame and suspension are redesigned is why none of the 2010 systems would fit. I bought the first one and installed it last week. What a difference. Not sure on the mileage increase but I know it will help. I noticed right away not having to press on the pedal as much to accelerate. Feels quicker and sounds amazing. Not too loud unless you step on it. Cruising at 70 its noticeable but not annoying. I drive about 150-200 miles a day and am on the phone a lot, was not a distraction to me. Huge 5" chrome tip looks sweet too. Its the single rear exit behind the tire. Very easy to install too. Call Thomas at Banks if you want one, I highly recommend them. Now the wait for a cold air intake and programmer. Pics to follow this weekend, truck is at body shop till tomorrow, truck was bumped into i a parking lot and pushed in the bumper.