That may just be the car breaking in and you getting better at driving it that is helping you get better mileage. All I've ever read about octane says that increasing it to more than what the manufacturer recommends will not improve fuel economy, but I know there can be exceptions.

I have also been using mid-grade gas since my first fill-up. Then again, our mid-grade here in the Denver area is 87 octane (regular is 85). Our grades are also all 10% ethanol.

 

Actually it recommends 87 or higher but not less than 87 I believe.

 

There have been discussions (here I think!) which suggest that on lean-burn engines the higher than suggested octane lets the engine move a little farther into an efficient state before knock sensors (or whatever) back off the timing or add another cylinder or something less efficient.

Or maybe being lighter by a few dimes helps! I think everyone should try it eventually and let us know what they see... Peter