I have a 2007 C4500 Crewcab Longbed dually 2 wheel drive with the Duramax/Allison setup. I pull a 15,000lb 5th wheel. I average 10 MPG but I usually have the cruise control at 60MPH. When I set it at 55MPH the MPG jumps up to 12.
I only use about 3/4 throttle on hills down shifting to lower gears as needed to prevent the engine from lugging.
Did you verify the mileage manually or go by an electronic indicator. The factory fuel use meters are notoriously off; some by as much as 5 MPG.
Slow down you will be amazed at how much better the MPG are. Enjoy the scenery...smell the flowers.
