As stated above, you only need it if you have had your car tuned for it. That means not just engine mods, but also you had your computer programmed for it.
In otherwords, if you have a stock car, you're fine with 85 octane. You will NOT see any benefit with running a higher octane.
Stupid me, just before the gas prices jumped had PCMForLess tune my computer to 91 octane. Sure, I get a bit of a performance boost and a bit better mpg... But the cost per gallon offsets the increase in MPG. If I could go back, I would.
