What I've found with my 18 is somewhat conflicting. Yes, under 60 mph is best, but not by much. If I travel at speeds less than 57 mph, the transmission hunts between 7th and 8th lowering the MPG. During the Winter, I drop 3-4 mpg pretty much across the board. But, running the AC has little to no effect. The 35 mpg trip I posted earlier was with the AC cranked the whole time.I have to agree with Bartley that the mileage is best below 60 mph. Much better mileage at 50 mph than at 70. I can get 27 mpg averaging 45 mph with the AC running.
Also have to agree with Bounty Hunter that the displayed mileage is not to be trusted for long trips. I thought I was cool by not resetting the A odometer for 32,000 miles until I realized the mpg indication jumped around so it wasn't accurate.
Two big factors affecting mileage I have noticed,
1) outside air temp below maybe 25F your mileage goes down about 3 mpg this may be due in part to winter blend fuel contains fewer BTUs
2) cranking up the AC costs about 3 or 4 mpg
I would say that idle time, along with "jack rabbit" acceleration, has the single most impact on lower MPG on a fully stock ECO. I've been known to shut my engine off at some of our 2+ minute traffic lights.