Originally Posted by 450Donn
My 04 4X4 ext cab scaled ready to go at 7500 pounds generally got 11 towing and usually got 18-19 non towing highway mileage. Just be aware the LLY was a short lived version and was probably the worst Duramax made. Many of then had overheating issues. That problem can be fixed for a price though. Personally I would look at late 05 LBZ or newer.
The problem isn't the engine with the LLY, it is the cooling system, specifically the size of the radiator, air intake and the fan. I was on the forefront of this issue back in '05 with a guy out of TX. They can be bandaided, but I agree, look for a LBZ+.
Regardless of the drivetrain, the remainder of the GM product 11 years and 117k later, is also disappointing. Time has come to sell my '05. Actually trade, as I'll probably make out better (or easier) than dealing with a private knowledgeable buyer.
To your mileage question, figure 10-12.