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

Even if your car's owner's manual says it requires premium fuel, you'll probably be OK if you run a little bit of regular.
Jason Statham gassing up an Aston Martin in LA. Bauer Griffin/GC Images via Getty Images

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Some cars require premium high-octane gasoline, others just recommend it, and most cars are fine running regular 87-octane fuel (it might even be lower at high altitudes). If your car normally takes premium but you’ve accidentally filled it with regular, or you’re just looking to save a few bucks on a fill-up, you probably won’t damage anything but don’t count on saving any money, either.

But what exactly will happen if you fill your premium car 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.

Premium vs. Regular Gas

Ben Affleck looks a little concerned on the phone here—maybe he’s calling the shop to make sure he picked the right octane for his Bentley. Bauer-Griffin/GC Images via Getty

Understanding the different grades of gasoline you can pick from the gas pump comes down to understanding octane ratings. Depending on where you live, premium fuel could be defined as 91 (mostly in Western states) or 93 octane (much of the U.S.) while regular is 87 octane in much of the country while it can be as low as 85 (in parts of Middle America and at high elevation).

Diesel, natural gas, kerosene, and other types of fuel you might see at a gas station are totally different—if you’ve just pumped one of those into your premium car, do not start it. If it’s running, shut it off. Get it towed someplace that can remove and drain the fuel tank.

But as long as you’re sticking with good old gasoline—octane references the likelihood of mistimed combustion. Basically, 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.

Cars that require premium typically do so because they’re turbocharged, high-compression, or have some kind of aggressive timing setup to extract maximum power.

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

If you have an old car that needs high-octane fuel, running 87 octane might make it start knocking before you can leave the gas station. It will sound bad and perform poorly, but even so, will probably escape permanent damage. Modern vehicles (like, post-1990) have much smarter onboard computers that can typically adjust their engine parameters accordingly when low-octane fuel is detected. However, they will likely make less power and be less efficient than they would be with their prescribed and preferred fuel octane. So if your car needs premium, you probably won’t actually end up saving money using regular because you’ll go through it more quickly.

Most modern engines can handle a fill-up with the wrong octane gasoline now and again. Andrew P. Collins

Does Premium Gas Give Better Mileage?

The short answer is no, if your car takes regular, you’re unlikely to see a noticeable benefit from using higher octane fuel. If it takes premium, you may see a decrease in gas mileage with regular, though.

Why Does My Car Require Premium Fuel?

If you don’t know if your car needs premium, check the owner’s manual first—that will tell you definitively. If you don’t have that, check to see if there are any stickers inside the gas-fill flap that say something like “premium fuel required.” If there’s nothing there, Google your car plus “premium fuel” but make sure you find a reputable source and don’t just go with whatever you see in the first Quora result.

A car that’s turbocharged, or a small car made for high performance, likely takes premium gas. Many highline vehicles (BMW, Mercedes-Benz, Maserati) will require premium but some cheaper cars do too (Honda Civic Si, older Nissan Maxima). The most bizarre premium-gas requirement we’re aware of is the Land Rover Discovery 1—the manual calls for premium despite that vehicle using a fairly primitive V8. That’s an especially big bummer considering you’re lucky to get mpg in the teens in one of those.

This McLaren is definitely dialed for high-octane fuel. Jerry Perez

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 buy an octane booster, which is pretty much a little bottle of super high-octane fuel to balance out the lower stuff.

A fuel station's pricing.
Most fuel stations offer at least a few different types of gasoline. DepositPhotos

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.
Premium gasoline is more expensive than regular. DepositPhotos

Does old gas reduce fuel mileage and performance?

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.

Do turbocharged cars need premium fuel?

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.

Watch: Premium vs. Regular Fuel Tested

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


Andrew P. Collins Avatar

Andrew P. Collins

Executive Editor

Pro car critic since 2012. Andrew’s also been an off-road tour guide, repo man, and Baja co-driver, among other things. Lifetime car nerd, amateur tinkerer, very slow casual racer.