I went up to Loudon, TN this weekend to get some things done and also to go to a truck meet in Knoxville. I headed home last night. From Knoxville, TN to Calhoun, GA on I-75 I put the cruise control on 70 mph and never went faster. I also never went slower than about 55 mph (road work area for about 4 miles). I was running the 275 Spartan tune and managed to get 19.7 mpg for that trip. That is hand calculated.
That is with fresh Air Dog fuel filters, new lighter wheels (20x9 BMF Novakanes), 35" tires, and 3.73's. There was also a pretty strong head wind.