I purchased a 2002 1500HD 4X4 in July of this year. I got 7MPG with the first two tanks of gas. I returned it to the Dealer for service. They found a calibration fault in the computer. After fixing the fault I got 10MPH (city). I took it back to the dealer and was told nothing could be done until it had about 3000 miles on it. OK. I took a 400 mile trip pulling a trailer with 2 ATV's on it and got 10MPG. I have dueled the exhaust with 2 FloMasters and 3" pipe out the back. I have installed a K&N air filter and still only get between 11 - 12MPG city. I do not have a heavy foot or make allot of quick start and stops. So I would have to say that I am very dissapointed in the gas mileage of the 6.0L engine.