There is somehow a misunderstand of the concept of Octane. It's NOT a rating of how much energy is in the gas - it is a measure of RELUCTANCE to combustion. In other words, the higher the octane rating, the more difficult it is to ignite. High compresion engines require high octane to prevent "pinking" - pinking being the remature ignition of the gas/air mixture. Thus many high performance engines requires high octane gas or gas additive cal "octane booster"

So, unless the engine is designed to require hi-octane gas, it's a waste of money. Even some engines that "recommend" high octance will run perfectly great with regulars - modern engines are all computer controlled, especially the ignition timing and can retard it if it senses pinking or premature ignition if you use low octane gas - and with no harm to the engine. My Infiniti Q45 (8-cyl high compression) recommend 91 octane. But in the life of the car I've always used 89 and the computer seem to adjust to it. I tried 87 but it seem to be out of the range of it's adjustment and pinks a little. I own the car for 10 years and when I sold it to get my HCH, it was in prestine condition and the engine purrs like a kitten. btw, I can oil every 10,000 miles - all done by the dealer - much to their disgust who continually badger me to change oil every 3K miles.

Bottom line - high octane gas do not give you better mileage.