Gasoline is primarily composed of octane. With that out of the way: the octane rating of gasoline only has a tangential relationship to the chemical octane. The octane rating is a measure of the resistance of gasoline and other fuels to detonation (engine knocking) in spark-ignition internal combustion engines.It is so named because a fuel composed of 100% iso-octane has an octane rating of 100.uels which are less resistant to detonation have an octane rating lower than 100, fuels that are more resistant to detonation have octane ratings greater than 100. Since ethanol is less explosive and more resistant to detonation than iso-octane, it has an octane rating greater than 100.