I know this might seem pretty absurd, but hear me out on this.

I'm getting better performance and mileage out of moving up to 89 octane from 87 in my non-SC V6. Why? Because it's so damn hot in Phoenix, AZ that temps in the engine bay rise to the point where pre-detonation of the gasoline is easy even on my very clean (internally) engine. By running 89 the gasoline is now resisting detonation until the time that the engine was designed for instead of causing pinging and low power like it was doing for a few weeks. Come winter it'll be no problem running 87 octane again but it makes sense to me at the moment to run 89 when the daytime highs are running 110+F.