Gasoline Octane Ratings in Different Parts of the U.S.

Don't know about east vs west; but at higher altitudes, the thin air causes slower burning and the lower octane will speed up the burning. I picked
up this info a long time ago, but I think I have it right.

edit:The word "elevation" would have been preferable to "altitude".

flyboy804 - Hmmm. You may be on to something. I WAS in an area of high altitude, when I noticed the difference.

I think that I have found the answer:

Link to Octane Article

The only thing that confuses me is the part where it says that in dry climates, you need MORE octane, but less in high altitudes. I was in an area of high altitude, and very low humidity. (Grand Canyon, and Southern Utah)

The Federal Trade Commision says on it's website:
The Low-Down on High Octane Gasoline

If you, Phoenix, had been elsewhere in the world than in the USA (and Canada, I think), you even would be more surprised:
only in the US they use the CLC method, while elsewhere RON-octane are used. [e.g. 91 RON octane is equivalent to 87 CLC octane.]

A little plus and a little minus; and I guess the minus wins out.

Without checking a single link, here's my understanding. Higher octane ratings are need to curb detonation in the engine. Higher air pressure yields higher combustion chamber pressure which leads to detonation, thus the higher altitudes don't require the same octane ratings as sea level.