| Wikipedia:
For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 11 kPa/km (1 inHg). An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet.
Fuel mixture needs to richen as altitude gains. Carburetors needed re-jetting for even slight changes. Octane is all about anti-knock. Pinging or pre-ignition is a bad thing. I think running low-grade is risky if ping is detected under acceleration.
My car loves Shell V-Power - yum!
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