If you got a 3500 with a 6.0 gas engine, you'll not be getting any good mileage any time soon. Doesn't matter if it's got 4.10 or 3.73 gears. I got a 2010 2500 HD with a 6.0  6 speed hydra-matic and 3.73's. truck has good power, pulls pretty good for a small block in 4th and 5th gears, 6th is pretty usless while towing. I use the manual mode and 5th when towing. It gets about 14 mpg running empty and something like 11, towing 6000#. My old 8.1/Allison will come close to doing that good. You can get a "Westers" or "Wait for me" tune that will increase power and drivability, but you'll have to use premium fuel and your mileage will probably drop even more.... You got what you got, want better mileage, find a good used early 2001-2003 Duramax diesel.... The 6.0 gasser is a thirsty critter!!

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