Octane rating is a measure of a fuel’s ability to resist knocking or detonation in an internal combustion engine. It is a standard measure of the performance of gasoline fuel. The higher the octane rating, the more resistant the fuel is to knocking or detonation. The octane rating of gasoline is determined by comparing the detonation resistance of the fuel to a mixture of isooctane and heptane. The test is done in a laboratory under controlled conditions, and the rating is expressed as a number. Regular unleaded gasoline typically has an octane rating of 87, while premium unleaded gasoline has an octane rating of 91 or higher. High-performance engines or engines with high compression ratios require higher octane fuels to prevent engine knock, which can cause damage to the engine.

RON, MON, AKI, and R+M/2 are different methods of measuring the octane rating of gasoline:

• RON (Research Octane Number): measures a fuel’s ability to resist knock under laboratory conditions that simulate high-speed, low-load driving
• MON (Motor Octane Number): measures a fuel’s ability to resist knock under laboratory conditions that simulate lower speed, higher-load driving
• AKI (Anti-Knock Index): is the average of the RON and MON ratings and is the most commonly used measure of octane in North America. It is calculated by averaging these two values and is displayed on the gas pump as the octane rating.
• R+M/2 (Road Octane Number or Pump Octane Number): is the average of RON and MON ratings, and is commonly used in the United States and Canada to describe the octane rating of gasoline sold at the pump.