Do any cars really need premium gas?

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What is octane?

In recent years, car manufacturers have been requiring or recommending premium gasoline (a high-octane grade of fuel) for use in more of their vehicle models. The difference in prices between premium and lower octane grades has also increased.

Octane ratings are measures of fuel stability. These ratings are based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The octane number is actually the simple average of two different octane rating methods—motor octane rating (MOR) and research octane rating (RON)—that differ primarily in the specifics of the operating conditions. The higher an octane number, the more stable the fuel. Retail gasoline stations in the United States sell three main grades of gasoline based on the octane level:

Regular (the lowest octane fuel–generally 87)
Midgrade (the middle range octane fuel–generally 89–90)
Premium (the highest octane fuel–generally 91–94)

Some companies have different names for these grades of gasoline, such as unleaded, super, or super premium, but they all refer to the octane rating.

A gasoline pump showing different grades of gasoline and octane ratings on the yellow labels

Source: Stock photography (copyrighted)

The large number on the yellow gasoline pump octane label is the minimum octane rating. (R+M)/2 Method on the label refers to the octane testing method used, where R is Research Octane Number and M is Motor Octane Number.

Of the 18 isomers of normal octane (C₈H₁₈), octane gets its name from the 2,2,4-Trimethylpentane compound, which is highly resistant to auto-ignition. This iso-octane has been assigned the reference value of 100 for testing purposes. The extremely unstable normal heptane (C₇H₁₆) molecule is the 0 octane reference fuel.

How does the octane level affect my vehicle?

Engines are designed to burn fuel in a controlled combustion. A flame starts at the spark plug and burns throughout the cylinder until all of the fuel in the cylinder is burned. In comparison, spontaneous combustion, also called auto-ignition, detonation, or knock, happens when rising temperature and pressure from the primary combustion causes unburned fuel to ignite. This uncontrolled secondary combustion causes pressure in the cylinder to spike and causes the knock to occur.

The competition between the intended (controlled) and unintended (spontaneous) combustion causes the energy from the burning fuel to disperse unevenly, which can cause damage and place high pressure on the engine’s piston before it enters the power stroke (the part of the cycle when the piston’s motion is generating power).

Normal combustion in a gasoline engine cylinder

Spontaneous combustion in a gasoline engine cylinder causing engine knock

Preignition in an engine cylinder

Before electric computerized ignition was widely used, this knocking commonly occurred and could cause significant engine damage. Most modern engines have sensors to detect knocking. When detected, the computer delays the initial spark, which causes the controlled combustion to take place at a point when compression is not at its highest point. Although this eliminates the knock, it can cause the engine to run less efficiently.

A similar undesirable condition is called pre-ignition, when the fuel ignites on its own before the spark ignites it. Modern engine computers minimize this condition by controlling the timing of valves and fuel injection; however, this control mechanism can also come with a fuel-efficiency or emissions penalty.

How is octane measured?

The standard means of testing octane is with an octane testing engine. This test is similar to the way the mass of an object can be determined by comparing it to objects (references) of known mass on a balance scale. Primary Reference Fuels (PRF) of precisely known octane are formed by combining iso-octane, heptane, and other well-known standards such as toluene. These PRFs are used to bracket a given fuel sample to determine the pressure at which similar knock intensities are observed. This measurement is taken by adjusting the octane engine’s cylinder height, which changes the compression ratio/pressure in the engine until the knocking reaches a specific intensity level.

The (R+M)/2 you see on the label refers to the average of the research octane number (RON) and the motor octane number (MON) ratings. To determine the RON, the fuel is tested under engine idle conditions with a low air temperature and slow engine speed. To determine the MON the fuel is tested under the more stressful conditions of higher air temperature and engine speed.

Historically, RON and MON were determined on separate testing machines specifically configured for each test. Current designs (see image below) allow the same engine to perform both tests. Despite this flexibility, many testers still prefer to use more than one machine with each specifically set up and calibrated to perform either RON or MON tests.

An octane testing engine

Last updated: November 17, 2022

Is premium gasoline really better for luxury vehicles?

Is high-octane gas worth the price? See car engine pictures.

©iStockphoto.com/asiseeit

With gas prices fluctuating and the economy in an unpredictable state, drivers are constantly looking for ways to save money at the pump. The most obvious way to get a little savings is to opt for regular unleaded gasoline and instantly save around 20 cents per gallon over the premium stuff. In most states premium gasoline is defined as gas having an octane rating of 91 or 92 — regular typically has an octane rating of 87. But will using a lower-grade gasoline actually hurt your car’s engine? This is an especially important question if you own a luxury car, because the manufacturers of most luxury vehicles recommend premium gas.

The answer to this question is somewhat complicated, but we’ll start with the easy part. The first thing you should do is check your owner’s manual. It’s entirely possible that the manufacturer of the car doesn’t even recommend premium gas. According to USA Today, some luxury cars from Ford and GM, such as the 2009 Lincoln MKS and the Cadillac STS, have been specifically manufactured to use regular unleaded fuel. If your car happens to be one of these examples, not only will you get no advantage from using premium fuel, but it may actually be harmful to the car’s engine in the long run.

Here’s another helpful hint: If your car does take premium, look to see whether the manufacturer «recommends» premium fuel or if it «requires» it. If it’s only a recommendation (as it will be in most cases), you can safely use regular unleaded fuel; however, you may take a small hit on performance. We’ll discuss the topic of performance in a little more detail later within this article. If premium unleaded fuel is a requirement, of course, you have to pay the extra money for premium fuel every time you fill up. This is because the car’s engine is precisely tuned for that grade of gas.

High-octane gas isn’t necessarily better than regular grade. It isn’t somehow more pure and it doesn’t go through a superior refining process. It doesn’t even keep your engine cleaner, as some people seem to believe. Premium fuel is just gas that contains a mixture of hydrocarbons that are slightly less combustible than those found in lower octane gas. This might seem odd, since cars use internal combustion engines that rely on the combustibility of gasoline to make them go. So, why would you pay extra money for gas that doesn’t ignite quite as well as less expensive gas?

The answer has to do with the way in which expensive high-performance engines, the kind that you often find in most luxury cars, are manufactured. Find out why that matters on the next page.

Premium Gasoline and Engine Knock
Premium Gasoline and Engine Performance

Premium Gasoline and Engine Knock

Engines that have a high compression ratio, like this race car engine, can benefit from high-octane fuel.

©iStockphoto.com/Petr Nad

Most internal combustion automobile engines, whether they’re four, six or eight cylinders, operate on a four-stroke cycle known as the Otto cycle. The four strokes are: intake, compression, combustion and exhaust. To put it in the simplest of terms, each of the vehicle’s pistons moves up and down within a cylinder. As the piston moves to the bottom of the cylinder, a mixture of fuel and air flows in. The piston then moves upward, toward the top of the cylinder, compressing the air and fuel mixture as it does so. Just as the piston reaches the top of the cylinder, that cylinder’s spark plug ignites. The spark creates a small, controlled explosion that forces the piston to the bottom of the cylinder. In the final stroke of the cycle, the piston moves upward to push the exhaust gas out of the cylinder. Once the exhaust gas has been pushed out, the entire cycle begins again. For a much more in-depth look at how an internal combustion engine operates, you may want to read How Car Engines Work.

As long as this process works as described above, the engine runs smoothly. But occasionally the pressure of the piston itself will cause the air and gas mixture to ignite prematurely during the compression cycle, creating a smaller, less powerful explosion. This is called preignition and it’s the cause of engine knock, the erratic rattling or pinging sound you may occasionally hear underneath your car’s hood. A little bit of engine knock isn’t necessarily bad for your engine, but it’s not desirable, either. It means that your engine isn’t running as efficiently as it could be, and left unchecked, it could eventually cause damage. Engine knock reduces your car’s performance, too, so you definitely want to avoid it. How, you may ask? Well, low-octane gas is more likely to ignite under the pressure of the piston alone, so it’s also more likely to produce engine knock.

Does this mean you should always use high-octane gas? Not necessarily. It really depends on the compression ratio of your engine. This is the ratio of the volume within the cylinder when the piston is at its lowest point to the volume within the cylinder when the piston is at its highest point. The higher the compression ratio, the more compressed the air and fuel mixture becomes and the more likely it is to ignite before it’s supposed to due to pressure alone. Cars with a low compression ratio don’t need premium gas because there’s little danger of the air and fuel mixture igniting improperly. But high-performance engines, which have a high compression ratio, are more prone to preignition and can truly benefit from premium fuel. This would include the engines in most luxury cars.

Even so, premium gas isn’t always necessary for these engines. We’ll find out why on the next page.

Should You Use Premium Gas in Your Car?

Manuel Carrillo III | Capital One

Article QuickTakes:

What is premium gas?
Why do some cars, trucks, and SUVs need premium fuel?
Can you put premium gasoline in any vehicle?
Can you mix premium and regular gas?

Is premium gasoline worth the extra money? The answer isn’t a simple yes or no—it depends entirely on the vehicle you’re filling up at the pump. The higher per-gallon prices, octane ratings, and the word «premium» might suggest this gas blend is better, but that’s only true under certain circumstances.

Understanding whether your car, minivan, truck, or SUV will truly benefit from premium gas can help save you significant cash throughout the ownership of your vehicle.

What Is Premium Gas?

«Regular» gasoline typically has an octane rating of 87, «midgrade» is 88 to 90, and «premium» is usually between 91 and 94 octane. Premium is used primarily by performance engines to help them reliably produce maximum power.

At some gas stations, premium gasoline may also contain a greater level of detergent additives intended to remove deposits left inside an engine during its operation.

Manuel Carrillo III | Capital One

Why Do Certain Cars Need Premium Gas?

To produce power, an internal-combustion engine uses pistons to compress a combination of air and atomized fuel just before a spark ignites it. The resulting explosion pushes the piston down, rotating the crankshaft and sending that output through the transmission to the wheels.

Performance engines typically squeeze the air-fuel mixture more forcefully. (This is known as a higher compression ratio.) Under these conditions, regular fuel is more susceptible to «knock,» or «pre-ignition,» which means the gas spontaneously ignites while it’s being compressed, before a spark plug actually fires. Too much knock can damage an engine.

The higher the octane number, the more knock-resistant gasoline is, and the safer it is for use in a performance vehicle. Many vehicles display a recommended octane rating on the gas-tank door (or in the owner’s manual) that indicates what gas blend will deliver the most power for your vehicle. The required rating, meanwhile, shows the lowest octane you should use in your car, as a lower-octane fuel could hurt the engine.

Can You Put Premium Gas in Any Car?

The short answer is yes. The longer answer is that fueling with premium typically won’t benefit the engine much.

For an engine to take advantage of the anti-knock properties of premium gas, it has to be designed to do so. On a modern vehicle, the computer controlling the motor recognizes the presence of high-octane fuel and adjusts performance accordingly.

If you put premium gas in a car that doesn’t call for the expensive stuff, it won’t affect performance, burn cleaner, or improve your fuel economy. So unless the manual suggests premium fuel, the rule of thumb is to avoid spending that extra cash at the pump.

Can You Mix Premium and Regular Gas in Your Fuel Tank?

There’s nothing wrong with mixing premium and regular gas in the same tank. In fact, this is how midgrade gas is formulated at the station. Rather than drawing from a separate storage tank for each octane rating, gas pumps actively mix high-octane fuel with lower-octane fuel to create midgrade gas.

TAGS premium gas

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Benjamin Hunting is a writer and podcast host who contributes to a number of newspapers, automotive magazines, and online publications. More than a decade into his career, he enjoys keeping the shiny side up during track days and always has one too many classic vehicle projects partially disassembled in his garage at any given time. Remember, if it’s not leaking, it’s probably empty.