2003 2500HD 4x4 extended cab I get 11.6mpg.
That was on the tires that were on the truck and before sea foam and oil change.
Tires were 305/70R16 cooper discover stt with little tread left. Just put on a set of 285/75R16 BFG all terrain ko's. Also ran some sea foam through the intake with a fuel injector cleaner in the tank and going to give it a good oil change. I'm hoping to see 13mpg, but I'm not holding my breath. Lol.