I personally have a 2007 Dodge with the 6.7 Cummins. I have all the emissions stuff removed. When driven like a car it will run around 18-20 mpg locally. I have gotten 22-24 mpg on long trips. When pulling a load, which it does often, it will range anywhere from 10-16 mpg depending on the load. The truck has 326,000 miles on it. Engine and trans are original. T-case was replaced at 320,000 miles due to operator error.
I service several 6.6 gm's in my shop. Many are in the '05-'10 range. Several have 100,000+ miles with one in particular, an '07 that has 230,000 on it. He uses it hard. Drive train is original. He runs 16-18 mpg empty and 10-14 mpg loaded. The others are all in that range too.
Casey in SD