Much has been said, in the various forums, about the mileage of the Duramax trucks. I have owned trucks with the LB7, LBZ, and LMM engines--all of them were 2500HD, Crew Cab, with 4WD. I can honestly say that I have not noticed an appreciable difference in mileage from one engine/truck to the other. I personally do not buy a 3/4 ton, diesel powered truck, with expectations of getting high MPG. I do know these are awesome trucks right off the shelf when referring to power, torque, and speed. If you temper your driving habits, I am sure you will be pleased with the Duramax performance, to include mileage.
2009 GMC Sierra, 2500HD, Crew Cab, 4WD, D-MAX
2009 Chevy Malibu LT, V6
2005 Jayco 323RKS, 3600W Onan Generator