In all my cars i've always used 87 Octane to fill up. The other morning i wasnt paying attention and put in 92, and i seem to have slightly higher gas mileage than usual...is this normal? or just a fluke...
In theory, if you modified other things to take advantage (timing, compression, etc) you could, all other things being equal, make a little more power with a given amount of 92 than 87. Which is the same as saying make the SAME amount of power while consuming slightly less 92 than 87. Which would translate to better mileage with the same driving style.
But since you didn't, and the car won't do it for you, it is probably a fluke. Plus, since 92 costs 10-15% more than 87, you'd have to get more than 'a little' better mileage to come out ahead.
Gas mileage is something you have measure, not judge. When it seems to be different, it sometimes is and sometimes isn't. You have to measure gas mileage by dividing the miles driven by he amount of gas used between fill-ups.
And don't forget factors like running the A/C, passenger load, tire pressure, and driving patterns.