Anything greater than 91 octane is very likely a waste of money, as 91 is what the engine is designed to run optimally on.
If you alternate fill ups between 93 octane and 89 octane, the blend you achieve in the tank will average out at 91. i.e., Fill your tank with 93 octane, when it gets down to about 1/2 refill with 89, run it down to 1/2 and fill with 93, etc., etc. This strategy would get you the optimal performance for the least amount of money.
Personally I use 89 octane nearly all the time. I only blend in some 93 when the weather is very hot, which is when the engine is most susceptible to "knock", and most likely to suffer reduced performance as the engine managment system dials back the ignition timing to prevent the knock. This is when having higher octane fuel is most helpful.
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