What Is Octane?
A primer on octane’s role in gasoline and its impact on vehicles.
Jun 19, 2026
Each day, nearly 40 million U.S. drivers refill their vehicle, typically choosing from three types of fuel: regular (87 octane), midgrade (89-90 octane) and premium (91-94 octane).
Octane ratings are measures of fuel stability, according to the U.S. Energy Information Administration. In the U.S., gasoline is marketed with an octane value calculated as an average of RON (octane tested under normal driving conditions) and MON (octane tested under more rigorous conditions) and designated on fuel pumps as (R+M)/2. This value, known as the Anti-Knock Index (AKI), is indicates the octane in gasoline that consumers see on fuel dispensers.
The different grades are related to the types of vehicle engines that require a different level of octane in gasoline. Premium gas can be used in all vehicles that run on gasoline, although it may not deliver better performance if it's not required. Premium is typically required in many luxury and high-performance vehicles. Mid-grade is not required in any vehicles.
E15, or 88 octane, is a blend of 15% ethanol (most fuel is 10% ethanol) and 85% gas. It is approved for use in all motor vehicles model year 2001 and newer; most vehicles on the road today can use E15.
Depending on a vehicle's engine design, octane plays a role in engine performance and measures the ability to resist auto-ignition, or knock. As engine compression increases, so does the amount of power and efficiency generated by a given amount of fuel. The higher the octane, the greater the fuel can withstand compression and resist knock.
An internal combustion engine compresses an air-fuel mixture in its cylinders, thereby raising the mixtures’ temperature and pressure. The air-fuel mixture is ignited with a spark during compression, and the resulting combustion releases heat energy that ultimately powers the vehicle. Knock can occur at a sufficiently high temperature (a consequence of compression) in the engine cylinders. Long-term, knocking reduces a vehicle’s fuel economy, robs the engine of power and causes engine damage.
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