Mixing 87 and 91 Gas: Is It Safe and What Octane Do You Get?
The Short Answer
Yes, you can mix 87 and 91 octane gasoline. The two grades are made from the same basic fuel components, they blend completely in the tank, and combining them will not harm your fuel system or engine. The result is simply gasoline with an octane rating somewhere between the two, depending on how much of each you added.
The qualification that matters is your engine's octane requirement. If your vehicle runs on regular, adding some premium does no harm but usually brings no meaningful benefit. If your vehicle requires premium, a tank that is partly 87 delivers less than the required octane, so treat it as a temporary situation and drive gently until you can refill with the correct grade.
What Octane Actually Measures
An octane rating describes a fuel's resistance to knock, also called detonation or pinging. Knock happens when part of the air-fuel mixture ignites on its own from heat and pressure instead of burning smoothly after the spark plug fires. Higher-octane fuel tolerates more compression and heat before that uncontrolled ignition occurs.
Octane is not a measure of energy content or cleanliness. A gallon of 91 does not contain more power than a gallon of 87, and it is not inherently a "better" fuel. It is simply more resistant to knock, which only matters if the engine is designed to take advantage of that resistance.
At US pumps, the number shown is the Anti-Knock Index, often printed as (R+M)/2. It is the average of two laboratory measurements, the Research Octane Number and the Motor Octane Number. This is why US octane numbers look lower than those posted in many other countries, which typically display the Research number alone.
What Octane You Get When You Mix Grades
When you combine two grades, the resulting octane is roughly the volume-weighted average of the two. Blending is not perfectly linear in a chemical sense, but for practical purposes the estimate is close enough to be useful.
Approximate result of mixing 87 and 91 by proportion; Mix | Approximate resulting octane; All 87 | 87; Three parts 87, one part 91 | About 88; Equal parts 87 and 91 | About 89; One part 87, three parts 91 | About 90; All 91 | 91
In practice, the proportion depends on how much fuel was already in the tank. If you have a quarter tank of 91 and fill the rest with 87, most of the tank is now regular, and the octane will sit much closer to 87 than to 91.
Mixing grades is also routine at the pump itself. Many US stations do not store midgrade separately; the dispenser blends regular and premium to produce it. That is one practical illustration that the grades are fully compatible.
Why Mixing Is Physically Safe
Regular and premium gasoline sold in the US are both blends of hydrocarbons, and most contain a similar share of ethanol, commonly up to 10 percent. They do not separate into layers, react with each other or form deposits because they were combined. Detergent additives required in US gasoline are present across grades as well.
This compatibility applies to gasoline grades. It does not extend to mixing gasoline with diesel, or to putting high-ethanol blends such as E85 into a vehicle that is not designed as a flex-fuel model. Those are different fuels with different requirements.
How Your Car's Fuel Requirement Changes the Answer
Whether mixing matters depends on what the manufacturer specifies for your vehicle. That information is typically in the owner's manual and often on a label inside the fuel filler door. Specifications can differ between engines in the same model line, so check by model year and powertrain rather than assuming one answer covers every version.
Effect of a mixed 87/91 tank by fuel requirement; Manufacturer guidance | Effect of adding some 91 to 87 | Effect of adding some 87 to 91; Regular (87) | Safe; typically no meaningful gain | Not applicable; 87 is already correct; Premium recommended | Safe; may restore some performance or efficiency | Safe; engine may reduce timing, with possible small loss of power or economy; Premium required | Safe; moves closer to the correct grade | Below specification; drive gently and refill with premium soon
Cars designed for regular
If the manual calls for 87, the engine is calibrated to run without knock on regular fuel. Raising octane gives the engine resistance it does not use. Adding premium, or mixing it in, will not damage anything, but it generally costs more without a measurable return in power or fuel economy.
Cars where premium is recommended
"Premium recommended" means the engine can safely run on lower octane but is tuned to perform best on premium. Modern engines use knock sensors that detect the early signs of detonation and retard ignition timing to protect the engine. With a mixed or regular tank, that adjustment may slightly reduce power and efficiency. Whether the difference is noticeable varies by engine and driving conditions.
Cars where premium is required
"Premium required" is a stronger statement. These engines, often high-compression or turbocharged designs, are built around the knock resistance of premium fuel. A knock sensor can still compensate for a partial tank of lower octane, which is why an occasional mistake is unlikely to cause immediate harm. However, repeatedly running below the specified octane, especially under heavy load, high temperatures or towing, increases the risk of knock that the system may not fully prevent. It may also become relevant if a warranty claim involves fuel-related engine damage.
What to Do in Common Situations
You accidentally added 87 to a car that requires premium: There is no need to drain the tank in most cases. Drive moderately, avoid hard acceleration and heavy loads, and top up with premium as soon as practical to raise the average octane. You want to save money on a premium-recommended car: Mixing grades or using regular is generally safe. Pay attention to how the car drives and whether fuel economy changes, then decide whether the savings justify any trade-off. Your regular-fuel car got a partial fill of premium: Nothing needs to be done. Continue using regular at the next fill-up. The station is out of your usual grade: Buy enough of the available grade to reach a station with the correct fuel, keeping the guidance for your vehicle's requirement in mind. You hear persistent pinging or knocking: Switch to the specified grade. If the noise continues on the correct fuel, have the vehicle inspected, since knock can also stem from mechanical or sensor problems.
Regional Differences in Grade Labels
Pump octane labels are not identical across the US. In many areas, premium is sold as 91, while in others it is 93. In some higher-altitude regions, regular may be sold as 85. Because the posted numbers vary, the useful reference point is the octane rating your vehicle's manufacturer specifies, not the name of the grade.
Lower-octane regular sold at altitude reflects that knock tendency decreases in thinner air for some engines. Many modern engines with electronic controls are designed around a specific octane regardless of elevation, so the manufacturer's guidance still applies, particularly for turbocharged engines.
Common Misconceptions
Premium cleans the engine better: Detergent standards apply across grades, and some brands add more additives to all of their grades. Octane rating alone does not indicate cleaning ability. Mixing grades damages the fuel system: The grades are fully miscible and chemically compatible, so the mix itself causes no harm. Higher octane always means more power: Power gains only appear when the engine is designed or able to adjust timing to use the extra knock resistance. You must run the tank empty before switching grades: Simply refilling with the desired grade is enough, as the octane averages out with each fill.
Bottom Line
Mixing 87 and 91 gas is safe and produces fuel with an octane rating roughly proportional to how much of each grade went into the tank. For cars designed for regular, the mix is harmless but usually unnecessary. For premium-recommended cars, it is an acceptable compromise with a possible small performance trade-off. For premium-required cars, a mixed tank should be a short-term exception, followed by a return to the specified grade.
