I don't think I could get mid 20's if I put the truck in neutral and pushed off a cliff. Best I ever got was 22 MPG running stock tires (265/70R17) at 70 mph on cruise and not towing. I can consistantly get 18-19 mpg on the highway when changing speeds and about 15 mpg for in town. If you put your foot in it often, then don't expect that kind of mileage.
2008 GMC 2500HD LMM SLT CC SB 4x4 Z71
Buckstop front bumper, WARN M12000 winch, PIAA 520 ATP driving / PIAA 510 fog, Westin Platinum Series nerf bars, Pace Edwards Bedlocker tonneau cover, Cognito UCA leveling kit and HD Idler Pivot Assembly, Cognito Pitman and Idler steering braces, Bilstein 5100's, LT285/70R17 BFG AT KO tires, 17 x 9 XD Series Crank Wheels, 15% Tint Driver/Passenger windows, GM Tow Mirrors, DDM Tuning HID lights 6000K 35W