Regular vs Premium Gas: Octane, Required vs Recommended

Octane rating is the measure of a fuel’s ability to resist knocking or pinging during combustion; it is not a measure of how much energy the fuel contains or how “clean” it is. Use the octane grade your manufacturer requires. If premium is only recommended, fueleconomy.gov says the benefits in normal driving are minimal. If premium is required, using it is worthwhile.

Key takeaways

• Octane is about knock resistance. In the U.S., regular is typically 87, midgrade 88 to 90 and premium 91 to 94.
• “Premium required” and “premium recommended” mean different things. Check your owner’s manual and the fuel door.
• Using lower octane than required can cause the engine to run poorly and can damage the engine and emissions control system over time.
• Higher octane in an engine that does not need it generally does not buy extra efficiency.

What octane is

Knock is the metallic pinging that happens when fuel ignites too early in the cylinder. A higher octane number means the fuel resists that better. fueleconomy.gov states that in the U.S., standard levels are 87 for regular, 88 to 90 for midgrade and 91 to 94 for premium. Most vehicles need 87 octane, and some require a higher grade. Ethanol itself has an octane rating of approximately 109 and is commonly blended into U.S. gasoline at up to 10%.

Why some engines need higher octane

Engines with a higher compression ratio, or that use supercharging or turbocharging, tend to require higher octane because they put more stress on the fuel. Turbocharged engines are discussed in start-stop and turbocharged engines. The manufacturer sets the requirement after designing the engine for that fuel.

Required vs recommended

• Required. The engine is designed for higher octane. fueleconomy.gov says using a lower octane fuel than required can cause the engine to run poorly and can damage the engine and emissions control system over time. Older vehicles may produce audible knocking; newer ones adjust timing and still lose engine power and fuel economy.
• Recommended. The engine can run on regular and the maker says premium performs best. According to fueleconomy.gov, for engines where premium is merely recommended, the benefit in normal driving is minimal, and higher octane may slightly improve performance and gas mileage during severe duty such as towing a trailer.

The wording in your owner’s manual is the authority. If it says regular is acceptable, premium is optional.

Does premium cost more than it saves

Premium usually costs more per gallon because, as fueleconomy.gov notes, fuel components that boost octane are generally more expensive to produce. fueleconomy.gov’s conclusion is that if premium is required it is worthwhile, and if it is only recommended, the cost increase is typically higher than the fuel savings. The size of the price gap changes with the market and the place, so check current prices from the EIA, which publishes weekly regular, midgrade and premium prices.

The arithmetic is straightforward. A vehicle at 25 MPG uses 600 gallons in 15,000 miles, so each 10 cents per gallon of price difference between grades is $60 a year. At 30 MPG it is 500 gallons and $50 a year per 10 cents. That is simply the cost; whether a performance benefit is worth it is a personal judgment.

How Gallonly treats grades

The cost on this site uses the price of the fuel type listed with the vehicle’s EPA rating: premium price for a premium-rated vehicle, regular price for a regular-rated one. The methodology page describes this. If you fuel differently from the rating, for example regular in a vehicle where premium is only recommended, adjust the inputs in the fuel cost calculator.

Common mix-ups

• Octane is not a cleaning agent. fueleconomy.gov frames octane as knock resistance. Fuel cleanliness is a separate topic and is not conveyed by the octane number.
• Midgrade is an intermediate octane level. The same rule applies: use it if your vehicle calls for it.
• E85 is a different fuel. It is for flex-fuel vehicles; see E85 and flex-fuel vehicles. Diesel is separate again; see diesel and DEF basics.

For ways to reduce fuel spending that do not involve changing grade, see how to lower your fuel costs. To see how prices enter our calculations, read how annual fuel cost is calculated.