[SD987]

For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 1 inHg (11 kPa/km). 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.

Octane ratings can vary greatly from region to region. For example, the minimum octane rating available in much of the United States is 87 AKI and the highest is 93. In the Rocky Mountain (high altitude) states, 85 octane is the minimum octane and 91 is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical combustion engine draws in less air per cycle due to the reduced density of the atmosphere. This directly translates to reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but if carried back to sea level the fuel may cause damage to the engine.