well heres what i learned, dont listen to the owners manual or anyone else. i was using premium and pushing like 10.5 miles per gallon (before tune up). tried 89 seeing as everyone keeps saying these trucks are not tuned to burn 93. well my mileage dropped to like 9.5 miles per gallon since filling with mid grade gas. screw regular and mid grade gas I'm sticking with 93. and I know that its the gas because I just had a tune up and have all new motor-craft plugs and a new coil, when i got the truck back i was pushing around 11.5 but then put in the mid grade and watched my mileage meter plummet
What brand of gas were you using for the low, mid and premium during your testing? Were they all the same brand? Also, how consistent was your driving during your testing and how long did your testing last? There is a noticeable difference in engine performance over time when using branded gas which is better (i.e. Shell, Texaco, BP, etc) vs "no-name" gas (i.e. everything else).
In my daily commute, I can vary my driving style very little but can range from 12 MPG average for a tank of gas all the way up to 14.5 MPG for a full tank of gas. Also, keep in mind that it takes the ECU in the truck time to adapt to the new grade of gas, at least one tank full if not more. The changes you saw in performance, both the improved and decrease, can be attributed to the ECU not having enough time to fully adapt to the different grade of gas.
Again, I am not saying that higher octane gas does not help. But if you give your truck the proper amount of time to adapt to the new fuel (while keeping your driving style as consistent as possible) you will find that the actual improvement of the new gas is not as much as you would think, and therefor potentially not worth the extra cost unless you get a tune that is specifically designed for the higher octane.