My '07 Ram gets the best mileage on the 91 canned tune. I tried 2 tankfuls of 87 (truck hated it and I hated the truck
), 3 tankfuls of 89 (ran better but not that great...MPG dipped a bit) and since then 92 octane. From the 89 tune I've seen a 4+ mpg improvement on the highway and I don't cruise slowly, normally running around 72 - 74 MPH.
So, for me, the 91 tune worked best but your mileage may vary.
I'd test a bit with different tunes and octanes under similar conditions and do your mileage calculation by hand.
The MPG counter in the truck is a system that measures MPG in real time and will vary depending on speed, load, ambient temp, etc....I've seen mine change with almost every mile when I reset it, drive around town, then hit the freeway. It took almost 60 miles of hwy driving for it to reach the best MPG reading.