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My Acura TSX has a 10.5 : 1 ratio and the manual says to use no less than 87 PON, but recommends 91 PON. There is an extremely subtle difference in drive feel between running regular and premium, with a little more pep when accelerating. Fuel economy is the same regardless of octane. The difference between the octanes are most easily observed by looking at timing advance; with the higher octane having a significant increase in timing, perhaps up to 10 degrees. I never get codes about detonation because the engine timing adjusts automatically to the fuel used. I've even run as low as 85 PON when staying at higher elevations (5000ft).
Sometimes I alternate fuel fills between regular and premium, with the resulting octane falling somewhere between the two. Plus grades of fuel in Oregon and Washington are simply 60% regular mixed with 40% premium. You can often mix your own and save a very small amount of money.
My motorcycle has an 11.5 : 1 ratio, revs to 14,000 RPM, and Honda recommends 87 PON. Perhaps it runs a little rich to prevent detonation? It doesn't have an O2 sensor, but is fuel injected.
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