Had to make two 600+ mile (one way) trips to eastern Colorado recently. First trip ran at speedlimit (75 on interstate, 65 other hiways). Second trip ran at 2000 rpm in 6th gear (64 mph). Used Cruise both times, winds were similar, empty truck both times, and about 250 miles are interstate the way I go. First trip got a hand calculated 17.8 MPG. Second trip got a hand calculated 21.9 mpg. I now run at a max of 64 mph no matter what. I have even seen my loaded mileage go up from 9.5 or so to 10.5-11.5 depending on winds.