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If you don't change the timing of your vehicle, I can see where some will say that increase in octane sees little results. BUT, if you were to use 91, then adjust timing to where it just stops pinging, then you should see enough of an increase in FE to justify the expense. I did the math and found that by doing just what I said, 91 octane is more than worth using for increased FE.
With 85 octane, I get 54 mpg (+5 deg timing). With 91 octane, I get 60 to 61 mpg (+10 deg timing). It costs me $.065 per mile in fuel with regular and $.060 per mile with premium. My break even mpg would be 57 mpg. Anything over 57 on premium makes premium fuel the least expensive.
Using regular 85 octane, it will cost me $142.19 per month in fuel over 2200 miles.
Using premium 91 octane, it will cost me $133.08 per month in fuel over 2200 miles.
87 octane puts the numbers about midway.
I can't make it any clearer.
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