Can You Mix 89 And 93 Gas? | Safe Octane Math

Mixing midgrade and premium gas is safe for most engines and creates a tank near 91 octane.

Yes, your car can run with 89 and 93 gasoline mixed in the same tank. Gasoline grades are not separate fluids that fight each other once they meet. They blend, and the octane rating in your tank lands between the two grades based on how many gallons of each one you added.

The only catch is your owner’s manual. If your engine calls for 87, a mix of 89 and 93 is more octane than it needs. If it calls for 91, the mix can work well when the amounts are close. If it requires 93, half 89 and half 93 may leave you short.

Mixing 89 And 93 Gas In Your Tank The Right Way

Octane is a rating of a fuel’s resistance to knock, not a measure of power packed into the fuel. Knock is the sharp, uneven burn that can happen when fuel ignites too early inside the cylinder. Modern engines often correct for mild knock by changing timing, but that can reduce snap and mileage.

When you mix two gasoline grades, the simple pump-side math is a weighted average. Equal amounts of 89 and 93 land near 91 octane. More 93 pulls the tank higher. More 89 pulls it lower.

• 5 gallons of 89 plus 5 gallons of 93 gives about 91 octane.
• 3 gallons of 89 plus 7 gallons of 93 gives about 91.8 octane.
• 7 gallons of 89 plus 3 gallons of 93 gives about 90.2 octane.

That’s why mixing can be handy when your car asks for 91 but the station sells only 89 and 93. It’s also why random splashing isn’t a great habit for engines that require a firm 93. The math may leave you under the rating printed in the manual.

What The Octane Number Actually Means

The yellow sticker on a U.S. gas pump shows the automotive fuel rating. For gasoline, that number is the octane rating, and the FTC fuel rating rule requires fuel sellers to determine and post it.

A higher octane fuel resists knock better. It does not clean your engine by default. It does not add horsepower to a car designed for regular fuel. It does not make every engine run smoother. The gain appears when the engine’s design or driving load calls for that resistance.

Required Versus Recommended Fuel

Owner’s manuals use careful wording. “Required” means the engine is built to run on that grade. Dropping below it can cause poor running, warning lights, or long-term strain. “Recommended” means the car can run on lower fuel, but the higher grade may help under heat, hills, towing, or hard driving.

FuelEconomy.gov gives the same practical split in its page on selecting the right octane fuel: buy midgrade or premium when your vehicle requires it; when premium is only recommended, the choice depends on your driving and cost.

So the blend question is less about whether 89 and 93 can mix. They can. The better question is whether the final tank rating meets what your engine asks for.

When The 89 And 93 Blend Makes Sense

The mix is most useful when you’re trying to land near 91. Many turbocharged or higher-compression vehicles ask for 91, but some stations in the U.S. sell 87, 89, and 93 only. In that case, half midgrade and half premium is a reasonable way to land near the grade your car wants.

It can also help if you accidentally put some 89 into a car that prefers premium. Topping off with 93 raises the tank average. If the engine requires 93 and you added a lot of 89, drive gently, avoid heavy loads, and refill with 93 soon.

Situation	What The Blend Does	Best Move
Car requires 87	89 and 93 mix is safe but usually wasted money	Return to regular next fill
Car recommends 91	Equal 89 and 93 lands near the target	Mix evenly or buy 91 when sold
Car requires 91	Equal gallons are usually fine	Track gallons so the tank stays near 91
Car requires 93	Half 89 and half 93 is too low	Use mostly or only 93
Accidental 89 in premium car	93 can raise the tank average	Top off with 93 and drive gently
Hot weather or towing	Higher octane can help engines prone to knock	Follow manual notes for heavy load
Trying to save money	Blend may cost less than all 93	Only do it if the final octane fits
Check engine light appears	Fuel grade may not be the only cause	Scan codes before blaming octane

How To Calculate The Mixed Octane

The pump math is easy. Multiply each fuel grade by the gallons added, add those numbers, then divide by total gallons. It won’t turn you into a refinery chemist, but it’s a sound way to plan a fill-up at the station.

Here’s the formula:

((Gallons Of 89 × 89) + (Gallons Of 93 × 93)) ÷ Total Gallons = Estimated Tank Octane

Say you add 6 gallons of 89 and 6 gallons of 93. The math is 534 plus 558, divided by 12. That equals 91. If you add 4 gallons of 89 and 8 gallons of 93, the result is about 91.7.

Why The Tank Won’t Be Perfectly Exact

Your tank may already contain fuel before you start. That leftover fuel counts too. If you had 3 gallons of 87 left, then added 5 gallons of 89 and 5 gallons of 93, your final tank is closer to 90.1, not 91.

Fuel chemistry can also vary by region and season. Ethanol is one reason. The U.S. Department of Energy notes in its ethanol fuel basics that ethanol has a higher octane number than gasoline and is used for blending. That doesn’t change the driver’s task: match the rating your vehicle needs.

What To Do After Mixing Gas Grades

Once the grades are in the tank, don’t drain anything unless you added the wrong fuel type, such as diesel in a gasoline car. Mixing 89 and 93 is routine gasoline blending at the driver level. Start the car, listen for odd knocking, and drive normally if the final rating fits your manual.

If the blend is lower than required, be gentle until the next fill. Skip full-throttle pulls. Avoid towing. Don’t climb long grades at high load if you can wait. Then add 93 when there’s room in the tank.

Gallons Of 89	Gallons Of 93	Estimated Octane
5	5	91.0
4	6	91.4
3	7	91.8
2	8	92.2
1	9	92.6

Will Mixing 89 And 93 Gas Save Money?

It can, but only in the right car. If your vehicle requires 91 and your station sells 89 and 93, splitting the grades can cost less than filling the whole tank with 93. The savings depend on the local price gap.

For a car that only needs 87, the blend is usually a pricey detour. You’re paying for knock resistance the engine wasn’t built to use. For a car that requires 93, saving a few dollars with too much 89 can cost you throttle response and may increase knock risk.

Simple Buying Rules

• If the manual says 87, buy 87 unless the engine knocks.
• If the manual says 91, an even 89 and 93 mix can work.
• If the manual says 93, don’t treat a 91 blend as the same thing.
• If the manual says premium recommended, test both grades across full tanks and compare cost per mile.

That last point matters because the pump price is only part of the bill. If premium costs much more and your car gains little or no mileage, regular or midgrade may make better sense when the manual allows it.

Clean Takeaway Before Your Next Fill

You can mix 89 and 93 gas. The blend won’t harm a normal gasoline engine just because the two grades share the tank. The final octane lands between the two grades, so equal gallons make a tank near 91.

The smart move is to match the finished blend to your manual, not to the highest number at the pump. Use the math when 91 isn’t sold, top off with 93 after a small mistake, and don’t undercut an engine that requires premium. That keeps the answer simple at the pump and safer on the road.