Your mileage will vary significantly depending on usage. The Duramax doesn't like stop and go. It'll do 15ish. I've towed a 3,000# trailer from CO to East TX and averaged 28mpg running 75mph. The trailer was a tandem flat bed so very little air resistance. Going back with a 3,500# tractor on it that had an enclosed cab, hence lots of air drag, I got half that. Pulling my 35' 15,000# 5th wheel (13.5' high) I get 10-11 at highway speed. Your DEF consumption will also vary A LOT. Under load it'll use way more than running easily. Hope that helps. Let me add that the Duramax is an awesome engine, truly amazing! It has no problem accelerating on on the grade up Wolf Creek pass towing that 15,000# 5th wheel. The temperature of the engine and transmission only climb 2-3 degrees.
