Mustang GT 20 mpg/no highway

Man, last week was reading somewhere that the 05 up 4.6 was actually factory tuned for premium, but the knock sensors are so effective, its not required...just for the heck of it, filled with premium...didnt feel any different(but wasnt going to the dragstrip either, just driving to work...). Weird thing was, yesterday when I went to fill up, had over 270 miles on the trip clock showing- normally always 230-something...the car has averaged 17-18 mpg like a clock on non-highway mileage forever, one tank of premium and it goes over 20 for the first time...hmm, tell ya what- premium next trip to the track if its pulling enough advance to pick up the mpg about 10%, it ought to be getting a bit more hp at full throttle I would hope.

last highway trip was 24.5 mpg @ 70 mph, will see what premium on the highway gives next time...27 possible?
That would be not bad for a high 13 second bone stocker...I was happy with 24

I have always ran 91 octane with a tune and CAI or not. I averaged mid 19's around town with or without the mods.
I swapped the rear gears to 4.10's and now average mid 18's.
But I don't drive conservative either. One thing I noticed over time was that if I drove more aggressive my mileage went up. Not lugging the car around I can only guess is why.
But awhile back I as well as others noticed our mileage took a drop for no reason. Then it went back up after about two months. Again for no reason.
We all run 91 octane too.
Our WAG was that they added ethanol to the fuel and didn't tell anyone. We live in a 15% or less does not need to be reported state. Then (guessing) they took it back out and our mileage went back up.
That is just a WAG, but it was funny all of us (about 6 cars) experienced the drop at the same time, and the rise back up at the same time.
?????

Rich Sr.

funny thing- on our way out west, tried a tank of hitest in the wifes 07 Avalanche, mileage dropped from 21 to 18(highway). guess its tuned for lower octane and the slower burn of hitest dont pull any more timing, so it drops MPG. The Mustang sure did better though