Octane Number Requirements of Vehicles at High Altitude SAE - 872160, written in 1987

Past tests of vehicles show that their octane number requirements decrease with altitude. (E10/E15 did not exist then and is worse now 2020)

As a result, gasoline marketers sell lower-octane-number (ON) gasoline in the mountain states and other high-altitude areas.
The current ASTM specifications, which allow reduction of gasoline octane of 1.0 to 1.5 ON per thousand feet, are based on CRC test programs run on 1967 to 1972 model vehicles.

However, many new 1987 vehicles are now equipped with sophisticated electronic engine systems for control of emissions and improvement of performance and fuel economy at all altitudes. (Does not work well for E10/E15 – JR)

Because these new systems could minimize the altitude effect on octane requirement,

Amoco Oil tested twelve 1984-1986 model cars and light trucks.

We found their ON requirements were reduced on average about 0.2 ON per thousand feet on an (R+M)/2 basis (RMON/1,000 feet).
We expect octane demand on gasoline suppliers in high-altitude areas to increase as these new cars make up a larger part of the vehicle population, and this could raise the cost of gasoline.