In general, you can physically mix 87 and 91 gasoline, but it’s not advisable as a routine practice. The blend will sit somewhere between the two octane numbers, and if your engine requires higher octane, mixing can reduce performance or trigger knocking.
Understanding octane ratings and what blending does
Gasoline octane is a measure of anti-knock resistance, not a direct measure of energy or cleanliness. In the United States, “87” and “91” refer to AKI (anti-knock index). When you blend fuels, the resulting octane is typically between the two original numbers, depending on the exact proportions and any ethanol content. Ethanol blends can also affect the effective octane rating and how the fuel behaves in your engine.
How octane blending works in practice
Mixing two fuels with different octane ratings mixes their characteristics in roughly a volume-weighted way. If you mix equal parts of 87 and 91, you might expect something near 89 AKI, but the exact result can vary with ethanol content and other additives. Higher ethanol content can raise the octane rating slightly but reduce energy per gallon and may influence how your engine runs.
Is mixing 87 and 91 gasoline safe for your vehicle?
Whether mixing is safe depends on your car’s requirements. Vehicles designed for regular 87 octane can usually tolerate a blend that stays above 87, but you won’t gain extra performance from adding higher-octane fuel. Cars that require premium fuel (often 91 octane or higher) may experience knocking, reduced power, or abnormal timing if the blended fuel delivers less-than-required octane.
What happens to performance and reliability?
When the octane rating of the fuel falls below your engine’s minimum, you may hear knocking or pinging, especially under load or when accelerating. Modern engines with knock sensors can adjust timing to mitigate knocking, but this can sacrifice power and efficiency over time. Conversely, using a fuel above what your engine needs typically does not improve performance and can be more expensive.
Practical guidance for motorists
Before considering a blend, here are practical considerations to guide your decision.
- Check your vehicle’s owner’s manual for the recommended octane rating (minimum and recommended). Do not deliberately dip below the recommended level.
- If you must mix due to fuel availability, ensure the resulting blend meets or exceeds the minimum octane your engine needs. A blend that falls below that threshold should be avoided.
- Be mindful of ethanol content. If the two fuels have different ethanol percentages, the mixture’s ethanol content will shift accordingly, which can affect compatibility with your fuel system and fuel economy.
- Understand that mixing is not a guaranteed way to improve performance. It may reduce efficiency or cause unwanted engine behavior, particularly in high-compression or performance-oriented engines.
In practice, the safest approach is to refuel with the grade your manufacturer specifies. Mixing should be considered only in cases of necessity and with awareness of your engine’s requirements.
What to do if you’ve already mixed
If you accidentally mixed fuel, you can continue driving if the resulting blend still meets or exceeds your vehicle’s minimum octane requirement. If the blend is uncertain or you notice knocking, refuel with the recommended grade as soon as possible and avoid aggressive driving until the fuel has been replaced.
Summary
Mixing 87 and 91 gasoline is physically possible, but not recommended as a regular practice. The final octane will typically lie between the two numbers and may not meet your engine’s needs, especially for vehicles that require premium fuel. Always consult your owner’s manual, consider ethanol content, and prioritize fueling with the grade your engine is designed to use. If in doubt, fill with the recommended octane to preserve performance and reliability.