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Past tests of vehicles show that their octane number requirements decrease with altitude. 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 vehicles are now equipped with sophisticated electronic engine systems for control of emissions and improvement of performance and fuel economy at all altitudes. 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).