Why are minimum octane levels different in some states?

[Bihili]

On a recent vacation we drove to the Grand Canyon and traveled thru 8 states.
The minimum octane level here in Missouri is 87 however in the states with higher elevation we found minimum octane level gasoline at 85.
One state was 86.

Is the difference because of elevation or just because it is different in each state or because of something else???

[Rick_L]

At elevation. with the same fuel octane and the same ignition timing, the tendency to detonate or ping is less. So where there's high elevation, they can back off slightly on the octane rating.

Thing is, at higher elevation you can run more timing and get some of the lost performance back - if the fuel octane remained the same. With EFI, you could tune the spark map that way and be good as you could at any elevation, and do it seamlessly.

So I'm guessing that the automakers aren't aggressive about the tune and the refiners can save a little money with the lower octane.

[hutchenc]

Rick pretty much covered it. Effective compression at higher altitude is simply lower so you don't need the same octane. Lived at high altitude almost my entire life...91 is the highest in CO and WY. When I lived in Montana, there was one station where you could get 93, but for the most part, it's 85, 87, and 91 at altitude.