Octane

A numeric rating of gasoline's resistance to detonation (premature ignition); the higher the octane number, the lower the chance of detonation. In the US, this figure is known as the Anti-Knock Index (AKI).

The minimum octane rating available in much of the United States is 87 AKI and the highest is 93. However, in the Rocky Mountain (high altitude) states, 85 octane is the minimum octane and 91 is the maximum. 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.

Although it is safe to fill up a car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, if carried back to sea level the fuel may cause damage to the engine.