I've been running an experiment, I drive about 200 miles round trip to my office once a month. Previous couple months been running 87 and just looking at the dash and it's been in the 14.3-14.8 range. Finally did a "real" test. Late last month did 87, last week did 89. Totally unscientific, just filling up at the same gas station, driving up and back and filling up again. Hand calc my truck is proud by about 0.5mpg. I got 14.8 on the dash, 14.3 hand calc on 87. I got 15.6 on the dash, 15.2 hand calc on 89. I'm going up again this week, but I've reinstalled my front plastic air dam(been removed since 600miles on truck, now going on 9k) and added a soft bed cover. Going to do 89 again, see how much it gains.
Trip Details: 30miles of 60-65mph, then about 60 miles of interstate running 70-75 mph, then driving through downtown Indy to the office.