I was in Colorado, and I saw gas stations serving 85 octane gas, along with 87 and 89. Also, I know that the altitude is around 5000 ft, compared to 1000 where I'm from. So, my guess is altitude, and pressure. Right?
G T
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In higher altitudes you can run octane that is lower than the ones in lower altitudes.
Higher altitude has lower air density so it actually lowers your compression because the engine draws less air, so you don't need as high octane to stop detonation.
http://en.wikipedia.org/wiki/Octane_rating