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Why Mixing Different Octane Gasoline May Destroy Your Car

Why Mixing Different Octane Gasoline May Destroy Your Car

Writing about different types of fuel is a daily practice on this blog. However, we haven’t answered every question yet. Therefore, we will use this blog to talk about what happens when you mix different types of gasoline that differ in octane ratings. Let’s start with a quick answer:

Mixing regular or mid-grade gasoline (87 – 89 octane) with premium gasoline (91 – 93 octane) won’t be harmful to cars designed to run on regular gasoline. However, mixing these gasoline types in cars designed for premium gasoline may result in engine knocking and damage to the fuel system depending on the car.

However, that certainly doesn’t answer the question wholly. Below, we’ll dive deeper into what happens when you mix regular, mid-grade, and premium types of gasoline. We’ll look at the effects this has on cars designed for regular gas, premium gas, and motorcycles. We’ll also look into what happens when you mix different ethanol levels and if there’s any benefit to mixing any of these types of gasoline. Read on!

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What Happens When You Mix Regular And Premium Gasoline?

First of all, it’s essential to understand that mixing different octane ratings results in different results depending on the octane ratings that you’re mixing. In the reasonably simple table below, we’ve shown you the octane level you create when you have a 50/50 split of each type of gasoline.

50%50%Resulting Octane Rating

Some people think that 87 octane is entirely different than 93 octane, or that 89 is different from 91, and so on. This is not the case. Assuming all other factors remain equal, you’re still filling your car with chemically identical gasoline besides the octane rating.

Mixing different octane ratings is not the same as, for example, mixing gasoline and diesel, which are two types of fuel that are refined entirely differently and will destroy the fuel system of your car within seconds.

Also read: The Best 93 Octane Gas Is Sold By These Retailers

Cars That Use Regular Gasoline

What happens after you (accidentally) mix two different fuel types depends mainly on what kind of car you’re using. Let’s first assume you mix mid-grade or premium gasoline with regular, unleaded gasoline in a car that’s designed for regular, 87 octane gasoline.

In this case, you’re very much in luck. Filling up a car with premium gasoline when the car is designed for 87 octane will not harm your engine or your fuel system. Furthermore, it will also not deteriorate the performance or lifespan of your car. In this case, the only thing you lost out on is saving a couple of bucks.

Some people even fill up their car designed for 87 octane with 91 or 93 octane all the time. This is also not advisable since filling up with premium, even though the car is designed for regular gas, does not give any benefits whatsoever. This is because the engine and fuel system aren’t intended to use the higher octane ratings.

In conclusion, don’t worry and fill up with 87 regular unleaded next time.

Cars That Use Premium Gasoline

If you filled up your car with regular, unleaded gasoline with an octane of 87, even though the car is designed for premium gasoline with an octane of 91 – 93, then it depends on a few factors what’s going to happen.

First, it’s important to know how much regular gasoline you put in the car. If you only put 3 gallons of regular unleaded gasoline in, even though you still had 30 gallons of premium gasoline, this will likely not affect the car much. A simple calculation tells us that your car is now filled with (10% x 87) + (90% x 91) = 90,6 octane gas. It’s doubtful this will cause your car any problems.

On the other hand, it’s possible you were running on empty, and you filled up with 87 octane completely. What happens then depends on the car you’re using and how old it is.

Older Cars

Typically speaking, putting regular unleaded gasoline in a premium gasoline vehicle will result in heavy engine knocking. This is the result of the misfiring of the spark plugs, which aren’t used to the lower octane gasoline. Eventually, this will destroy your engine and fuel system if not appropriately addressed.

If you hear heavy engine knocking after refilling (slight engine knocking is not a problem), immediately stop the car and call in assistance. The car will need to be drained of the fuel, and the fuel filter will need to be replaced.

Newer Cars

However, engine knocking these days only happens in older vehicles. Cars have gotten more modern fuel systems and engines in the past years. These modern systems can detect the octane rating in the gasoline and adjust the firing of the spark plugs accordingly.

However, it’s not entirely clear what cars can do this and which aren’t. Carmakers also don’t communicate this very clearly since they don’t want people to fill up their cars with gasoline that wasn’t specified in the owner’s manual.

Therefore, it’s still best practice to fill up with the type of gasoline mentioned in the owner’s manual.


For motorcycles, the same rules apply. If you’re using premium gasoline in a motorcycle designed for regular gasoline, you’re not very likely to run into problems.

However, if you filled up your motorcycle completely with regular gasoline when it’s designed for premium gasoline, you’ll most likely run into problems more quickly. It’s also unclear whether modern motorcycles have systems capable of detecting and adjusting for different octane levels.

If you fill up your motorcycle with regular gasoline when it needs premium, it’s best to drain it completely. Also, make sure to replace the fuel filter if your motorcycle has one.

Can You Mix Different Ethanol Levels?

Next, it’s essential to know if you can mix different ethanol levels of gasoline. As you may know, gasoline is sold with either 10%, 15%, or 85% ethanol. These are referred to as E-10, E-15, and E-85. We’ve already discussed mixing gasoline with E-85 in this blog: Can You Mix E-85 And Unleaded Gasoline? (Explained). Therefore, in this subheading, we’ll talk about mixing E-10 and E-15.

Whether or not mixing E-10 and E15 is a problem for your car depends on the car you’re using. At most gas stations, E-10 will still be the most provided type of gasoline. However, since 2011 E-15 has also been approved for sale in the United States. This is because ethanol is more eco-friendly in its production, and therefore the use of ethanol is supported.

Officially, the EPA approved E-15 for light-duty vehicles from 2001 onwards. This means mixing E-10 and E-15 in a car manufactured after 2001 won’t be a problem. However, if you’re using an older vehicle or one that’s classified as high-duty, it’s advised to use E-10 since this is what these vehicles were designed for.

Furthermore, if you read the owner’s manual of cars manufactured before 2011, you’ll notice that carmakers mention the car is suitable for E-10. This is because E-15 didn’t exist before that. It should be fine (as long as the car is made after 2001). However, if you want to be sure, follow the information in the owner’s manual.

Is There Any Benefit To Mixing Different Types Of Gasoline?

Finally, you probably want to know whether there’s a benefit to mixing gasoline with different octane levels. In other words, would you want to mix different types of gasoline deliberately?

Gas Mileage

First, let’s talk about gas mileage. Mixing different types of gasoline (that have different octane levels) will not result in a change in gas mileage. Since you’re still using gasoline with the same ethanol level, the energy density remains the same.

However, if you mix different ethanol levels, then the story changes. This is because ethanol provides more mechanical power but is less efficient. For example, E-85 (which confusingly enough has 51 – 85% ethanol depending on the region and the season) provides 20 – 30% fewer miles per gallon than E-10 does.

Mixing E-10 with E-15 won’t provide a noticeable change in gas mileage. However, mixing E-10 with E-85 or E-15 with E-85 will significantly decrease gas mileage. Please make sure to only mix these kinds of gas in Flex-Fuel vehicles and read the owner’s manual on necessary precautions before mixing these fuels.

Cost Saving

Another reason why people sometimes mix gasoline is that they want to save costs. Again, mixing different octane levels will not result in cost savings. Mixing 50% 87 octane with 50% 93 octane will only result in 91 octane that will most likely cost as much as directly filling up with 91 octane.

However, at first sight, it does seem that mixing E-85 with E-10 or E-15 has merit in Flex-Fuel vehicles. This is because E-85 is much cheaper to buy. We wrote a blog post discussing this question in detail and came up with some pretty interesting conclusions: Is E-85 Cheaper To Use Than Other Gasoline? (Explained)


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