What Happens When You Put Regular Gas In a Premium Car?

Your car may require premium fuel, but what happens if you run a tank of regular every once in a while?

how long does gas last
Laura Gangi Pond

It’s never a good idea to tempt the automotive maintenance gods, but sometimes there just isn’t a pump with premium fuel to be found when you need it. If you’ve ever found yourself in these situations, it can be stressful trying to determine what the impact on your vehicle will be. The good news is that, in most cases, nothing bad will happen.

The Drive’s crack informational team has been in this situation many times, and while we’re not scientists, we do have quite a few white-knuckle miles under our belts, spent skipping through the desert to find a better gas station. We’ve found that a tank of regular now and then won’t kill your engine, nor will it cause an immediate revocation of your car enthusiast card.

So, what will happen if you fill up with regular fuel? Will you cause damage to your engine? Stick with us while we take you through the intricacies of premium versus regular fuel and how each one affects your vehicle’s operations.

[Note: This article was originally published on Sep. 27, 2020, but was updated with new formatting and new information on 05/12/2021.]

Premium vs. Regular Gas 

A gasoline pump.
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Understanding the different types of fuel comes down to understanding octane ratings. Depending on where you live, premium fuel could be defined as being 91 or 93 octane (there are other places around the country with higher octane than 93, though), while 87 octane is considered regular.

Octane refers to the likelihood of mistimed combustion. Higher octane ratings mean there’s less of a chance that combustion will occur at the wrong time. The poorly-timed tiny explosions that take place are known as pre-ignition, or engine knock, because of the sound they produce.

Higher octane gasoline is designed to help prevent engine knock. This phenomenon isn’t typically harmful to your engine if it happens occasionally, but repeated engine knock can speed up wear and tear.

What Happens If You Put the Wrong Octane of Gas In Your Car?

In the old days, running 87 octane in a car that required premium fuel would cause the engine to start knocking before the car even left the gas station, but things are different today. If you mistakenly fill up with regular fuel, your car’s ECU will adjust engine timing and performance to work with the lower-octane fuel. In most cases, this is ok for the times when there are no premium pumps around, but it’s not ideal to run the cheapest fuel possible for extended periods of time. 

A Mazda engine bay.
DepositPhotos

Modern engines can handle a fill-up with the wrong fuel now and again.

Does Premium Gas Give Better Mileage?

The short answer is no, you’re unlikely to see a noticeable benefit from using higher octane fuel. Using the proper octane rating for your vehicle will help it perform as intended, which may have the side effect of producing better fuel economy, but it won’t be enough of a benefit to justify the use of more expensive fuel.

Why Does My Car Require Premium Fuel?

Depending on the type of vehicle you own, the requirement to use premium fuel could come from a number of factors. Your vehicle may have a turbocharged engine, which requires higher octane fuels to operate properly, or maybe you opted for a high-performance car with a high strung engine that operates with tight tolerances. Whatever the case, if your vehicle’s gas cap or owner’s manual tells you to use premium fuel, do it.

Pro Tips To Mix Premium Gas With Regular Gas

The Drive’s contributors have found themselves stuck without a premium fuel pump in the past, and can say that both driver and vehicle will survive a trip with regular gas. Mixing fuels of two different octanes will result in a tank of fuel with an octane rating somewhere between the two fuels, depending on the amounts of each.

That said, if your vehicle requires premium fuel, it’s a good idea to top it off with the good stuff as soon as you’re able. If you’re really worried about the engine developing a knock, you can add just enough regular fuel to get you to the next station, but be sure you have enough fuel to reach the destination.

If you’re really desperate, you can opt for an octane booster, which will bump the octane number in your current tank of fuel.

A fuel station's pricing.
DepositPhotos

Most fuel stations offer at least a few different types of gas.

Fuel Safety

You may already know that it’s stupid to smoke while filling your vehicle fuel, but there are other things to keep in mind at the pump:

  • Don’t try to top off your vehicle’s fuel tank. Clicking the pump on and off may give you the satisfaction of knowing the tank’s full, but it could lead to problems as fuel runs over into parts of your vehicle where it doesn’t belong
  • If, for some reason, you’re tempted to drain your fuel tank, do so safely. Work in a well-ventilated area and don’t store fuel near open flames. Also, this should go without saying, but don’t ever siphon fuel from your tank using your mouth
  • If you get gasoline on your skin or clothing, wash off as soon as possible. It can irritate your skin and cause rashes or other damage
A fuel station's list of available gasoline.
DepositPhotos

Premium fuel is more expensive than regular.

FAQs About Gas Mileage

You have the questions, The Drive has the answers!

Q: Do turbocharged cars need premium fuel?

A: In general, many manufacturers recommend using premium fuel for turbocharged vehicles. Check your owner's manual for specifics about your vehicle and the type of gas you should use.

Q: What is Top Tier gas? 

A: Top Tier is a branding and marketing name and term used by select gas companies, but it does not necessarily indicate premium gas. Top Tier's website says it is, "a fuel performance standard written by auto manufacturers to assist in keeping the engine cleaner, thus resulting in improved customer experience. Since auto companies are not able to dictate fuel regulations or fuel standards, the TOP TIER™ logo serves as an indication of a station where the fuel marketer is supportive and offers the vehicle manufacturers' request for higher standards in fuel.” 

Q: Does old gas reduce fuel mileage and performance?

A. According to Exxon, gas sealed in an approved container has a shelf life of about six months because it starts to break down and lose its combustibility. As it degrades, it might lose some of its potency, which could lead to minor levels of lessened fuel mileage and worse performance. Learn how to remove and transfer fuel with our guide, How To Dispose of Old Gas.

Watch: Premium vs. Regular Fuel Tested

In this video, Engineering Explained's Jason Fenske puts gasoline to the test.

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