I have an 05 CC 2wd Duramax/Allison. This is my fourth Duramax. By far the 03 got the best fuel mileage and ran just about as good as the LLY. I hae driven several LLYs and all run better than mine, local diesel tech says it happens and no real reason why. Running at cruise control 70 It will get a best of 14.7 and a headwind will knock it down below 14. I dragrace and tow a 42ft enclosed gooseneck hauler(14,000#). Normal mileage is 8.9 towing 67 mph.. Headwind will take it below 8.0 I check all fuel mileage by filling to the neck and useing the trip odo. The truck indicator is usually within .1 to .2 mpg of the actual. This truck has 20,000 miles on it and the fuel mileage has not improved with age as I have towed with it from 500 miles on. The LLY has been a bit of a disappointment. I have owned Chevy trucks all my life and am not about to change but this particular truck is not the best one I have ever owned. My LB7 trucks would get 10 towing the same trailer and have been as high as 11.5 on no wind level ground.