What the Octane Numbers on the Gas Pump Really Tell You

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If you have ever stood at a gas pump and wondered whether your car really needs that expensive button marked “Premium,” you are not alone. Gasoline pumps offer a small menu of numbers ranging from 85 to 93 and then politely wait for you to make a decision that could cost anywhere from a few cents to a dollar or more per gallon.

Those numbers are octane ratings. They do not tell you how much energy is in the gasoline, how “powerful” the fuel is, or how clean it will make your engine. What they primarily tell you is how resistant the fuel is to knock, also called pre-detonation. And understanding that distinction can save you money.

Let’s dive into what the octane numbers actually stand for and why it’s important to know which one you should be using in your vehicle.

What Does “Octane” Actually Mean?

A gasoline-powered engine requires three things to operate: air, fuel, and spark. Air is all around us, so it’s easy to get. It enters the engine and is mixed with vaporized gasoline. A spark ignites the fuel and air mixture, and the resulting combustion presses a cylinder down to rotate a crankshaft, producing the power that makes the car go.

Under certain conditions, some of the unburned mixture ahead of the flame front can become hot and pressurized enough to ignite on its own. Instead of one controlled combustion event, the engine gets multiple combustion fronts colliding inside the cylinder. That creates the characteristic metallic knocking or pinging sound heard as the vehicle rus, especially when accelerating. Usually referred to as “knock.” This misfire causes the piston to move prematurely or not enough, depending on when it occurs, resulting in incorrect engine timing.

Modern engines are generally pretty good at detecting and managing knock, but that doesn’t mean you want it happening constantly. Persistent severe knock can damage pistons, bearings, spark plugs, and other critical engine components.

Octane rating essentially tells you how resistant gasoline is to this unwanted self-ignition. Higher octane fuel is more resistant to knock. The more resistant to knock a fuel is, the harder it is to get it to combust. That creates a careful balance in fuel requirements between knock resistance and ease of ignition.

In general, gasoline engines in the U.S. are made to use fuels between 85 and 93 octane. Some require a specific rating while others are largely indifferent. The reasons for this are a combination of air mixture and engine compression. Higher air pressure (through induction, turbocharging, and so forth) often coincides with higher pressure before ignition in the engine. Higher pressure before combustion increases the chance of pre-ignition knock. This is why many turbocharged engines require higher octane fuel.

Decoding the Octane Numbers At the Pump

When at the gasoline pump, you’ll see anywhere from two to four options for octane ratings. These numbers are standardized by ASTM International for North America and enforced in the U.S. by the Federal Trade Commission, with the octane rating listed on the pump given as the Anti-Knock Index (AKI) number. Below that number, you may have noticed some pumps add the label “(RON+MON)/2” or “(R+M)/2”.

That’s the formula used to calculate the AKI rating for the fuel. The “R” or “RON” is the Research Octane Number, while the “M” or “MON” is the Motor Octane Number. Adding those together and dividing by two to get an average returns the AKI.

That means 87-octane gasoline isn’t 87 percent octane. It is a rating based on the fuel’s resistance to knocking compared with standardized reference fuels. This is why the numbers can be confusing when comparing American gasoline with fuel ratings used in other countries. Many countries advertise RON rather than the AKI number used in the U.S.

The test procedure for getting those RON and MON results is very similar. The RON is achieved by running an engine at low revolutions per minute (RPM), with valve timing varied throughout the cycle to increase the likelihood of inducing knock. The MON is tested the same way, but at higher RPM and with variable ignition timing added to further raise the chance of knock. Generally, the MON will produce a lower average result than the RON. The results of the RON and MON are then compared to a baseline and given a rating. Those two ratings are then averaged.

Why Do Some Vehicles Require Premium Fuel?

Higher octane does not equate to more power. This is probably the biggest misconception surrounding premium gasoline. Putting 93-octane fuel into a Toyota Prius, which was designed for 87 octane, doesn’t magically turn it into a NASCAR winner. If the engine’s computer isn’t designed to take advantage of the higher octane, you may notice absolutely no difference other than your wallet becoming slightly lighter.

However, there is an important exception. Some engines are specifically calibrated to take advantage of higher-octane fuel.

Many high-performance, high-output, or highly efficient engines frequently use higher compression ratios, turbocharging, supercharging, aggressive ignition timing, or combinations of these. Those characteristics can increase cylinder pressures and temperatures, making knock more likely. Using higher-octane gasoline gives those engineers more room to operate without encountering destructive levels of knock.

In those engines, premium fuel can allow the engine to produce its intended power and efficiency. Many turbocharged, high-efficiency engines require higher octane fuel to operate. This is much more common now, as newer vehicles use smaller, more compact, and more efficient engines that employ turbocharging, higher compression ratios, etc. This is true of vehicles that are not performance or sports-oriented, like pickup trucks, sport utilities, and minivans.

Does Higher Octane Help a Car That Doesn’t Need It?

This depends on the vehicle, but for the most part, higher-octane fuels are unlikely to benefit engines not designed to use them. Some vehicles can use higher octane to improve performance. Mazda, for example, often lists horsepower ratings alongside fuel octane ratings because the SKYACTIV engine family can vary valve timing more or less based on the amount of knock sensed. More knock likely means a lower-octane fuel, so the engine’s computer will adjust timing less often to prevent knock, thus reducing potential power output. And vice versa.

Most modern engines are designed to do something similar, though how much adjustment will depend very much on the engine’s design. The majority of hybrid vehicles, for example, will not benefit from higher-octane fuels because they use a less varied timing cycle to improve efficiency at the lower rpm rates at which they are most often running.

Is It Worth Saving Money By Going With Lower-Octane Fuel?

For most vehicles and the way we drive them, the answer is yes. Most of the time, paying more for the premium fuel isn’t going to improve your efficiency or clean your engine better. Most of the “cleaning” of your engine during operation, including the carbon buildup that is often talked about, happens despite the fuel, not because of it. The obvious caveat being that if your vehicle’s owner’s manual or the manufacturer recommends or requires higher-octane fuel, that is what you should use.

An efficiently burning engine free of knock will also burn nearly everything that would otherwise coat the cylinders and cylinder heads. The cleansers in fuel at the pump do very little cleaning inside the engine. Fuel additives, however, can have a bigger effect on carbon buildup, but are still limited as to how much they can accomplish.

The Short After the Long of It

The octane rating of fuel does not equate to its power output or performance level. It is a measure of how resistant the fuel is to early combustion under pressure, or knock. If your engine requires higher-octane fuel, you should pump high-octane fuel. Most engines do not benefit from raising octane levels unless the manufacturer specifically lists a benefit.

For these reasons, you can save money at the pump by using minimum octane fuel (usually 85 or 87). There is very little benefit to using high octane fuels in a vehicle that cannot utilize it.