I have a 2011 F-350 Diesel 6.7L that has around 400HP and 800 lbs. torque. It pulls my 39 ft. 5th wheel Montana effortlessly through any mountains. Mileage is on average 17 without and 11.5 with the RV in tow. As always, much of the mileage has more to do with your driving habits, however; no one mentioned that the differential is also a bit important when it comes to mpg, although some say it's insignificant. We have a 3.55, however; I know there are 3.73's that are getting almost 1-2 mpg's less with that engine. Just an FYI, the 2015's have the same engine, but have increased the HP and torque.