I got new tires and have been testing 87 versus 91 for the fuel to see if I can actually notice a difference. Manual says 87 but to use 91 if I want "higher performance" which I find kind of vague. Also the tire place didn't fill my tires enough, so my fiance put some more air in them which should help my mpg's out as well I am hoping.
I recommend that you avoid using 87 octane gas on hot Summer days. This is when you will lose the most power. I use 89 octane during the Summer and 87 when it is fairly cold outside.