My 1 ton 4 door 4x4 dually with a 6.7 cummins is getting about the same mileage as friends of mine with idnetical power train in 3/4 ton with single wheel 4x4.
My 6.7 was horrible when I first got it but now days the mileage is coming up. If there is no wind and I'm driving nice I get 17ish, the minute the wind comes up or I get all over it the mileage goes way down. This summer we took it on vacation to the mountains up pikes peak, instate, city and it averaged 16.4 the entire trip.
I had the RIGHT to remain silent, I didn't have the ability!