My moms truck is a '03 2500HD with 4.10 gears and it gets 12.5 MPG. I've heard the 6.0 L gets around 15 mpg with 3.73s but dont know for sure. Its a high revving motor, thats for damn sure. It isnt unusual to hit 5k rpm with it. I honestly think its a bitchin motor in stock form, so I dont know of any mods for it.