Can I Mix 87 And 93 Gas? | What Your Engine Gets

Yes, mixing regular and 93 gasoline is safe; your tank gets a mid-grade octane blend based on how much of each you add.

If you already pumped 87 into a tank that had 93, or you topped off regular gas with 93, don’t panic. Gasoline grades are made to burn in spark-ignition engines, and the octane number changes when grades blend. Equal parts 87 and 93 land at 90 octane.

The real issue is whether the final octane is enough for your engine, driving load, and the fuel label printed inside your fuel door or owner’s manual.

Mixing 87 And 93 Gas In Your Tank: What Changes

Octane is not a measure of power. It measures how well gasoline resists knock, the sharp rattle that can happen when fuel burns too early inside the cylinder. Higher numbers help knock-prone engines, mainly turbocharged, supercharged, or high-compression cars.

When 87 and 93 mix, they do not fight each other or separate into layers. The tank ends up with a blended octane rating. The easiest math is a weighted average:

• 5 gallons of 87 plus 5 gallons of 93 equals 10 gallons of 90 octane.
• 8 gallons of 87 plus 2 gallons of 93 equals 10 gallons of 88.2 octane.
• 2 gallons of 87 plus 8 gallons of 93 equals 10 gallons of 91.8 octane.

That blended number matters most when your car asks for 91 or 93. If your manual says regular unleaded is fine, the blend is fine too. If your manual says higher-octane fuel is required, a low blend may cause pulled timing, dull response, or knock under load.

What The Octane Number Means

In the United States, pump octane is the average of two lab ratings: research octane number and motor octane number. That is why fuel pumps often show “(R+M)/2” on the label. The FTC fuel rating rule explains how gasoline sellers determine and label that number.

So, 93 is not cleaner by default, and 87 is not weak fuel. They are different knock-resistance grades. Detergent quality, ethanol content, and brand additive package are separate issues. A good 87 from a busy station can beat 93 your car does not need.

When Mixing Regular And High-Octane Gas Is Safe

Mixing the two grades is safe when both are gasoline for passenger vehicles and both match the fuel type your car accepts. The main limits are simple: do not add diesel to a gas car, do not add E85 unless the car is flex-fuel, and do not ignore a “91 or 93 required” label.

For most daily drivers that call for 87, adding 93 only raises the tank’s octane. It won’t harm the fuel pump, injectors, plugs, sensors, or catalytic converter. You may not feel a change, because the engine was already tuned for regular.

For a car that recommends 91 or 93, mixing can be a practical stopgap. The engine can often adapt, but peak response may drop. Drive gently until the next fill if you had to add regular during a trip.

For a car that requires 91 or 93, treat that grade as the default. A small amount of 87 in a pinch is not an instant engine death sentence, but do not make it a habit. Top off with 93 soon and avoid hard throttle.

Situation	What The Blend Does	Best Move
Car requires 87	Raises octane above what the engine needs	Safe, but often costs more with no gain
Car recommends 91 or 93	May reduce peak response under load	Drive lightly; return to the listed grade next fill
Car requires 91 or 93	May fall below the required knock rating	Add 93 soon and drive gently
Equal 87 and 93 mix	Creates 90 octane gasoline	Works well for many midgrade recommendations
Mostly 87 with a little 93	Stays close to regular grade	Do not rely on it for 91-plus engines
Mostly 93 with a little 87	Stays near the higher grade	Usually fine unless the car is picky under load
Older engine with pinging	May quiet mild knock if octane rises enough	Use the grade that stops knock, then check tune-up items
E85 or diesel mistake	Not the same as mixing gas grades	Do not start the engine; get the tank handled

How To Read Your Fuel Door And Manual

Your fuel door may say “regular unleaded,” “91 recommended,” or “93 required.” Those words are not the same. Regular means 87 is the target. Recommended means the higher grade may give rated power or smoother response, but regular may be allowed. Required means the engine was built around that higher knock rating.

The FuelEconomy.gov octane page says U.S. regular gasoline is usually 87, midgrade is 88 to 90, and the top pump grade is 91 to 94. It also states that higher octane than the manual calls for does not improve performance or fuel economy in normal driving.

Do not trust badge names alone. “Sport,” “turbo,” or “luxury” can hint at higher octane needs, but the manual wins. If you bought the car used and the paper manual is gone, the automaker’s online manual by model year is the right place to check.

Why 93 Does Not Always Pay Off

High-octane gas is only worth buying when the engine can make use of it. The EIA octane grade chart explains that regular, midgrade, and higher pump grades all refer to octane rating, not extra power packed into the fuel.

If your car runs on 87 and does not knock, the money is usually better spent on oil changes, good tires, and fuel from a busy station. A station with steady traffic turns inventory often, which helps you avoid stale fuel.

Gallons Added	Blended Octane	What It Means
1 gal 87 + 9 gal 93	92.4	Still near 93
3 gal 87 + 7 gal 93	91.2	Often enough for 91-rated cars
5 gal 87 + 5 gal 93	90.0	A true midgrade blend
7 gal 87 + 3 gal 93	88.8	Close to pump midgrade
9 gal 87 + 1 gal 93	87.6	Still close to regular

What To Do If You Already Mixed The Grades

If the car calls for regular, just drive. There is no special drain, cleaner, or reset needed. The tank will burn down like any other fill, and you can return to 87 next time.

If the car recommends 91 or 93, listen for pinging under load. If it runs smoothly, finish the tank. If it feels dull, top off with 93.

If the car requires 91 or 93 and you added a lot of 87, do not floor it. Add as much 93 as the tank will take. If heavy knocking continues, stop driving and call a mechanic.

Simple Pump Math Before You Fill

You do not need perfect math at the pump, but a little tank sense helps. Half 87 and half 93 lands near 90. One-quarter 87 and three-quarters 93 lands near 91.5.

When You Should Not Mix Fuel Types

Mixing 87 and 93 is one thing. Mixing gasoline with the wrong fuel type is another. Diesel in a gasoline car can cause a no-start, smoke, misfires, and repair bills. E85 does not belong in a non-flex-fuel car.

Stop if you catch the mistake at the pump. Do not press the start button or try to dilute the tank. Call roadside help or a repair shop.

Clear Answer For The Next Fill-Up

You can mix 87 and 93 gas. Your tank will get a blended octane rating, and the engine will use it as long as the final blend meets the car’s needs. Regular-only cars do not gain much from 93. Cars that require 91 or 93 should get that grade whenever possible.

The safest habit is plain: follow the fuel label, buy from a busy station, and avoid paying for octane your engine cannot use. If you already mixed grades, match your next fill to the manual.