Putting regular gas in a premium car

While most gas-powered cars run on Regular fuel, some require Premium. Besides the heftier price tag, the octane rating, or grade, is the main difference. Regular gas generally has a grade of 87, while Premium ranges between 91 and 94.

The higher the octane rating, the more resistant the fuel is to early detonation.

Can you mix Premium and Regular gas? Combining fuel grades basically just alters the octane rating of what’s in the tank. In fact, many gas stations offer an 89-octane “Midgrade” option that’s merely a blend of Regular and Premium.

So, is it bad to mix gas grades?

In cars built for Regular, adding in some higher-octane fuel shouldn’t cause any harm.

However, putting Regular fuel in a Premium car will not only lead to reduced performance but may result in lasting engine damage.

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What Happens If You Put Regular Gas In A Premium Car?

Higher-compression engines, like those with forced induction (i.e., turbocharged), are designed to squeeze as much performance out of a motor as possible.

However, they also generate more pressure and heat, which may cause lower-octane fuel to ignite early, an issue that can lead to engine knock.

Engine knock, or “pinging,” essentially means the air/fuel mixture inside the combustion chamber is not igniting evenly. While the occasional knock is fine, persistent knocking can cause lasting damage to vital engine components, including the pistons, crankshaft bearings, and cylinder walls.

Not sure what engine knock sounds like? This short video from YouTube’s Motorcarnut gives a great example.

Due to Premium fuel’s higher-octane rating, it’s less likely to ignite early. This gives the vehicle’s Electronic Control Unit (ECU) time to optimize detonation and, thus, performance and efficiency.

Engine knock can stem from several things, like bad spark plugs or carbon buildup, but low-octane fuel is one of the most common.

What Happens If You Put Premium Gas In A Regular Car?

The benefit of Premium fuel is that it allows the ECU time to optimize detonation. However, a car built for Regular fuel doesn’t generate enough pressure or heat for early ignition to be an issue.

While filling up with Premium might sound like a way to pamper your car, there’s really no benefit if it’s made for a lower-octane rating.

Can you mix 87 and 91 gas? Sure, but you’re essentially just throwing away money. Instead, save yourself the 50 cents per gallon and opt for the gas grade your car is built for.

How Do You Know If Your Car Takes Regular Or Premium Fuel?

You’ll find the required fuel grade on a label inside the gas door. With time, though, this label may fade or fall off. Thankfully, the owner’s manual will also detail the octane rating your engine is designed for.

Finally, you can also provide your car’s VIN to a local dealer to find out the correct gas grade.

So, Can You Mix Different Grades Of Gas?

There are a lot of things you can do to extend the life of your vehicle. However, filling the gas tank with the wrong fuel grade is not one of them.

If you accidentally add Premium fuel to a car built for Regular, nothing bad will happen to the engine, and you can drive without worry. However, you also won’t get any benefits, so don’t expect improved fuel economy or more power.

Mistakenly adding Regular to a car made for Premium isn’t a crisis either, so long as it doesn’t happen often.

If you do notice the engine knocking after adding Regular fuel, you may be able to resolve the issue by pouring an octane booster into the tank.

Besides using the correct fuel grade, only filling up at Top-Tier gas stations is another way to extend your vehicle’s longevity.

CARS.COM — Premium gas is undeniably expensive. Gas stations typically charge significantly more for premium fuel, and filling up with it can take a serious toll on your wallet. With the price of gas on the rise, it’s tempting to only fill up with regular. Some vehicle manufacturers recommended using premium gasoline in their cars, though, and many drivers wonder whether doing so is necessary or if it’s just a way for oil companies to make extra money off the stuff that costs more per gallon.

Related: Got Bad Gas? Here’s What You Should to Know

Does using premium fuel really matter? Could failing to use premium gasoline void your warranty or damage your engine? The answers may surprise you. Keep reading to discover the truth before you next trip to get gas.

What If the Manufacturer Requires It?

When a vehicle manufacturer requires using premium fuel, it is because that particular vehicle’s fuel system is designed to work best with higher octane gas. Using regular gas in an engine that requires premium could void your warranty. That is most likely to happen if using regular causes severe engine knock or pinging (premature ignition of the fuel, also known as detonation) that damages the pistons or other engine parts. Using the wrong gasoline may cause other problems, like decreased fuel economy and engine performance.

For example, here is what GM says about the subject in an owner’s manual for a vehicle that requires premium:

“Use premium unleaded gasoline with a posted octane rating of 91 or higher. If the octane rating is less than 91, you could damage the engine and may void your vehicle warranty. If heavy knocking is heard when using gasoline rated at 91 octane or higher, the engine needs service.”

Note that this applies only to engines that require premium gas. Some manufacturers recommend premium gas but say that regular or mid-grade gas can be used instead. They usually warn that using lower-octane gas could reduce performance and fuel economy. When that happens noticeably, or if engine knock occurs, they advise to start using premium.

Differences Between Premium and Regular Gas

Premium gas is similar to regular unleaded fuel. Both are made from crude oil and are highly combustible. One of the most notable differences, though, is that premium has a higher-octane rating. Most premium fuel has a rating of 90 octane or higher. Octane is important when it comes to powering vehicles while preventing knocking. When gasoline and air ignite before they should, it causes an explosion known as pre-ignition, which causes a knocking sound. Octane refers to how well the fuel can resist pre-ignition. With an octane of 90 or higher, premium gasoline is less prone to pre-ignition or knocking. Premium fuel also contains detergents and additives that help engines run cleaner. The additives and detergents result in less pollution as well.

The Bottom Line

The computers that manage modern engines are able to adjust the ignition system to accommodate lower-octane gasoline — to a point. With regular gas, fuel economy and acceleration will likely deteriorate at least slightly. Because regular has lower octane, it is more prone to detonation. Burning regular in an engine designed for premium on a long-term basis or under heavy loads can cause engine knock, and that in turn can damage the pistons, valves or spark plugs. Due to the presence of knock sensors and the car’s ability to retard the spark timing, you might not hear knocking, but that doesn’t mean premium is unnecessary.

Regular can be used at least occasionally without repercussions in most vehicles designed for premium, but it’s a bad idea to make a habit of it. Bottom line: Check the owner’s manual. If the vehicle manufacturer says the engine requires premium, believe it. Don’t try to save a few cents per gallon by buying regular gasoline. Doing so could lead to much larger expenses in the future.

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