Fuel Mileage with TC

19,000-19,500 GVW, with 5.13 R&P gears, driving 70-72MPH I get from 6 to 8.5 MPG. Worst was in the 5 range driving 75, in a head wind, in hilly terrain. Nice that I can do that though :) Love that Duramax and those gears! Can get over 10 empty driving carefully (proved it to myself once, then went back to lead-footing it. :) ) I usually figure between 7 and 8. To the same idea that 6pac is showing, it is so incredibly speed sensitive when you are driving a 19,000 pound brick wall down the highway. If I could drive 60MPH I'd do oh so much better. Hope to test that someday when I am not in a hurry to maximize time away from work. After driving in Colorado for so long, and with a diesel to do it, I'd never go back to a gas truck. Diesel's were meant for the mountains and heavy loads.