I regularly fill up with 87 octane and only average 10mpg......that is most likely due to my front bumper and roof rack with no fairing though. Disappointing? Yes very much so, but those are the sacrifices I must make to drive such an amazing vehicle
That's nice. You can't get it in the big cities around here. I estimate about a 10% decrease due to ethanol and your 1.5 increase might not be far off. When I cared to calculate gas mileage, I tried to fill up the same way, same station and would keep track of 4-5 consecutive tanks.
It's sounds contrived, but my ford V8 will idle bad when the fuel blend changes. I had it towed to the shop 3 years in a row around the same time and they found nothing wrong.
If you haven't already, check for any pending O2 sensor codes, clean the MAF sensor, throttle body, unhook battery, add a can of K44, and monitor it for a while.