My modified 7.3L PS FWD 2002 model gets 21 on highway, driving pretty fast, 16.0-17.0 in town (I live in the mountains) and about 14-15 towing an 8000 pound load. When stock, the truck got about 3 mpg less across the board. The 6.0L ON AVERAGE seems to get about 1-2 mpg less than the 7.3L although that is highly variable. The Chevy Duramax owners I know, most of whom have modified their trucks in some way, get 14-15 in our area locally driving, and 18-19 on highway. On average the Cummins in the Dodge would do the best on fuel, especially when towing, although the folks I know who have them cannot equal my mileage although they do come close. I had a V10 250 before my diesel and it would get 11-12 in similar driving locally, 15 on highway and 8-10 pulling the same weight load. My opinion is those are big differences, but unless one drives A LOT it would take a long time to make a diesel work out on fuel economy alone. I personally could not justify buying one based on this fact, but to me there are several other reasons to buy a diesel.
John M