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From my experience driving service rigs for off road rally racing, a friend's Powerstroke 6.0 got about 12mpg towing a 5k pound trailer whereas Cummins trucks were routinely getting closer to 18.
I've been running my '09 2500 for about 3 months now and I am getting an avg of 18 mpg on the high way and a little lower in the city but I dont do much driving in the city either.