I've had my 2015 F250 6.7 powerstroke, longbed, crewcab 2 years and check my mileage almost every tank fill up, the old fashion way. Actual mpg about .2 better than computer. I average 13.5 to 14 mpg around town and up to 18 on interstate. Drove to Florida last year (1300 miles each way) averaged 15 to 18 mpg depending on how fast 65 or 80. Truck has bed cap higher than truck cab and truck weighs about 9000# and closer to 10,000# loaded for trip. Just came back from 10 week trip west, 12,540 miles and averaged 11.2 mpg by computer for the trip. Includes about 8000 miles towing 8400#, Rockwood 2906WS, travel trailer.
Hope this info helps.