Each day, nearly 40 million drivers pull up to a fuel dispenser to refill their vehicle, typically choosing from three types of fuel: regular, midgrade or premium. A fourth option is also available: 88 octane. In most areas of the United States, regular unleaded gasoline is 87 octane, midgrade is 89 and premium is 91 to 93. 

Most consumers purchase regular gasoline (86%) and 13% purchase premium. Midgrade only accounts for 1% of fuel sales, a steep decrease from 12% in 1997, and 88 octane is used by less than 1% of drivers.

In the United States, 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 the primary indicator of octane in gasoline that consumers see on fuel dispensers.

Many vehicle owners may not know how an internal combustion engine that runs on gasoline works, or why octane matters. Some may even think that selling regular to premium gas, because of their lower to higher price points, is a method of selling “OK gas” to “fancy gas.” In fact, 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. 

88 octane is a blend of 15% ethanol (most fuel is 10% ethanol) and 84 octane gas. The added ethanol to the 84 octane fuel pushes up the overall octane rating to 88. It is approved for use in all vehicles model year 2001 and newer; 90% of all vehicles on the road today can use this fuel, which is often called E15. Because ethanol is less expensive than gasoline, E15 fuel is often priced considerably less than other octanes.

If your vehicle is model year 2001 or newer, you can safely use E15 fuel, which is also known as 88 octane. While the lower price may make some consumers think it is a “lesser fuel, it adheres to the same standards as all other transportation fuels. Its lower price is a function of lower ethanol costs relative to gasoline. However, ethanol does have about one-third less fuel efficiency than pure gasoline. That means that E15 will deliver about 2% less fuel efficiency than E10 fuel. Conversely, ethanol-free gasoline, which is far less common, will deliver about 6% better efficiency.
 
One note of caution related to E15: It is only approved for motor vehicles model year 2001 and later. It should not be used for other vehicles, boats or gasoline-powered equipment (like lawnmowers, leaf blowers, etc.). It may cause damage and is prohibited by federal law.

Depending on a vehicle's engine design, octane plays a key role in engine performance and measures the ability to resist auto-ignition, which is commonly referred to as 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.

A gasoline 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.