Putting 91 Octane in a Car Rated for 87: What Actually Happens

If you fill up with 91 instead of 87, nothing bad happens—your car will run safely, and in most vehicles designed for regular gasoline you’re unlikely to see noticeable gains in power or fuel economy; you’ll mainly pay more per gallon. In some modern engines under heavy load (towing, high heat, steep grades) you might notice slightly smoother performance because higher octane can further suppress knock, but the difference is usually small.

What Octane Ratings Mean

In the U.S. and Canada, the posted 87, 89, 91, or 93 numbers are the Anti-Knock Index (AKI), a measure of a fuel’s resistance to engine knock (pre-detonation). Higher octane does not mean “more energy” or “more cleaning power”; it simply allows the engine to run more spark advance or higher compression without knocking. Modern engines with knock sensors adjust timing automatically: if they detect knock on lower octane, they pull timing (reducing power); if knock risk is low on higher octane, some engines can use a bit more timing. Whether that yields benefits depends on how the engine and its software were calibrated.

What You Might Notice When Using 91 Instead of 87

The following points outline typical outcomes when you put 91 octane in a vehicle that specifies 87 octane regular fuel.

• Engine safety: It’s safe. Using a higher octane than required will not harm the engine or void the warranty.
• Performance: In engines designed and tuned for 87, power usually doesn’t increase on 91. Some adaptive, high-efficiency engines may feel slightly smoother or marginally stronger in hot weather or under heavy load.
• Fuel economy: Most drivers will see no measurable MPG improvement. Independent testing (including AAA studies) has repeatedly found no cost-effective benefit in cars that are calibrated for regular fuel.
• Emissions and drivability: Emissions are typically unchanged. You may notice quieter operation if your engine was borderline knocking on 87 under certain conditions.
• Cost: Premium typically costs 10–20% more per gallon. Without a consistent performance or MPG gain, total operating cost rises.
• Additives: Detergent packages are brand-dependent, not octane-dependent. “Top Tier” certification indicates robust deposit control additives, whether the fuel is 87 or 91.
• Conditions that can magnify small benefits: Very hot days, steep climbs, towing/hauling, or aggressive acceleration can increase knock tendency; higher octane may help maintain full spark advance.
• Engines that adapt more: Some turbocharged or high-compression engines that “recommend” (not require) premium can deliver modest gains with 91 or 93, especially in warm climates.

Overall, the main difference most drivers will notice is at the pump: the higher price. Any performance or efficiency improvement in regular-fuel vehicles is usually minor and situational.

When Higher Octane Is Recommended or Required

Manufacturers use specific language in the owner’s manual. Here’s how to interpret it.

• Required/Only: If the manual says “Premium fuel required” (often 91 AKI or higher), use it. Many turbocharged or high-compression performance engines need premium to achieve designed power and to avoid knock-related stress.
• Recommended: If it says “Premium recommended,” the engine will run on 87 but may reduce power under load on regular. Using 91/93 can restore advertised performance and, in some cases, slightly improve efficiency, though the cost-per-mile may still be higher.
• Special conditions: Towing, desert heat, mountain climbs, or repeated hard acceleration can push even regular-fuel engines toward light knock. A tank of 89 or 91 during such trips can help.
• Audible pinging: If you ever hear persistent metallic “pinging” under acceleration with 87, step up in octane and have the engine checked if it continues.

If your vehicle specifies 87 with no recommendation for premium, stick with 87 for everyday driving; step up only if your usage or conditions warrant it.

What If You Mix 91 and 87?

Mixing grades is fine; octane blends linearly by volume. For example, half a tank of 87 topped off with an equal amount of 91 yields roughly 89 AKI. Your engine management will adapt in real time, so no action is required.

What If You Use 87 in a Car That Requires 91?

Occasional misfueling with 87 in a premium-required car usually isn’t catastrophic, but it’s not advisable. The ECU will likely retard timing to prevent knock, cutting power and potentially causing roughness; heavy throttle can still provoke knock. Drive gently, avoid high loads, and fill with the correct octane at the next opportunity. Repeated use of sub-spec fuel can, over time, increase stress and may affect warranty coverage if damage is linked to misfueling.

Practical Advice

The following steps can help you choose the right fuel and avoid paying for octane you don’t need.

1. Check the owner’s manual or fuel door: Follow “required” vs “recommended” guidance.
2. Prioritize Top Tier fuel: Choose stations that meet Top Tier standards for detergent additives at any octane.
3. Match ethanol requirements: Use the ethanol blend your vehicle supports (e.g., E10 is broadly approved; E15 is approved for most 2001+ vehicles in the U.S., but verify your manual; never confuse octane with ethanol content).
4. Listen and observe: If you hear knocking on 87 under load, try 89 or 91; if it stops, the higher octane is beneficial under those conditions.
5. Evaluate cost vs benefit: Any performance or MPG gain from 91 in an 87-rated engine is usually small; the added cost often outweighs it.
6. Skip “octane boosters”: They’re rarely cost-effective and can be inconsistent; buying the correct pump octane is simpler and more reliable.
7. For rentals or unfamiliar cars: Check the fuel door sticker; when in doubt and performance matters, 89 can be a safe middle ground.

These habits help ensure reliable operation, control costs, and keep deposit buildup in check without paying for unnecessary octane.

A Quick Note on Fuel Types

Octane rating is not the same as fuel type. Never put diesel in a gasoline car or vice versa, and don’t use E85 unless your vehicle is flex-fuel capable. If you’re outside North America, note that posted numbers are often RON, where 95 RON roughly equals 91 AKI, and 91 RON roughly equals 87 AKI.

Bottom Line

Using 91 instead of 87 in a car designed for regular is safe but usually not beneficial enough to justify the extra cost, except in specific conditions (heat, heavy load) or in engines that recommend premium. Always follow your owner’s manual, choose reputable Top Tier fuel, and base octane choice on your vehicle’s requirements and driving conditions.

Can you switch between 87 and 91 gas?

Yes, you can switch between 87 and 91 octane gas, and modern engines with knock sensors will adjust to the change in octane, so there’s usually no negative effect or immediate harm to your vehicle. However, if your car is designed for 91 octane, using 87 can lead to reduced performance, lower fuel economy, or a knocking sound from the engine, especially under heavy loads. Always check your vehicle’s owner’s manual for the recommended octane rating to ensure you’re using the correct fuel for optimal performance and engine health. 
Why You Can Switch

• Knock Sensors: Opens in new tabMost modern cars are equipped with knock sensors that detect engine knocking or pre-ignition, adjusting the ignition timing to compensate for lower octane fuel. 
• Engine’s Ability to Adapt: Opens in new tabThe engine’s computer can adjust to different octane levels to prevent engine damage. 

When to Be Cautious

• Engine Requires Premium: If your car’s manufacturer specifies premium (91 octane or higher) fuel, using lower octane fuel (87) can cause issues. 
• Reduced Performance: You might notice a decrease in acceleration and power when using lower octane fuel than your engine is designed for. 
• Engine Knocking: Using the wrong fuel can lead to spark knock, a high-pitched pinging or rattling sound. 
• Heavy-Duty Use: In situations like towing or driving in very hot weather, the higher octane of premium fuel offers better protection for high-compression engines. 

What to Do

1. Check Your Owner’s Manual: This is the best source for determining the minimum or recommended octane rating for your specific vehicle. 
2. If Using Higher Octane: Switching from 87 to 91 octane is generally fine and will not harm your engine. 
3. If Using Lower Octane: If your car requires 91 octane, you can switch to 87, but it’s best to do so only occasionally, for daily driving, and not as a habit. 
4. If You Filled Up By Mistake: If you put the wrong octane fuel in your vehicle but haven’t started it yet, contact a mechanic immediately. 

What happens if you accidentally mix premium and regular gas?

Mixing premium and regular gas is generally harmless, creating a fuel with an octane rating somewhere in between the two. For modern cars designed for regular gasoline, nothing significant will happen, though for premium-required vehicles, a noticeable drop in power and fuel economy may occur, potentially leading to engine pinging or long-term issues over time. For high-performance engines, turbocharged vehicles, or older cars, it’s best to stick to the recommended fuel to avoid engine damage. 
If Your Car Requires Regular Gas

• No harm done: Mixing mid-grade or premium gas with regular is not detrimental to your engine and will not cause damage. 
• Wasted money: You’re essentially paying more for a higher octane fuel than your engine needs, which offers no benefit in terms of performance or efficiency. 

If Your Car Requires Premium Gas

• Reduced performance: You might experience a noticeable decrease in engine power, acceleration, and fuel economy. 
• Engine Pinging: Modern cars with knock sensors can adapt by adjusting the engine timing and fuel injection to compensate for the lower octane. However, older cars without these advanced systems may exhibit engine knock (pinging), which is harmful and can cause damage over time. 
• Potential long-term damage: Consistent use of regular fuel in a premium-required engine could lead to increased engine wear and a higher repair bill in the future. 

Important Considerations

• Engine requirements: Opens in new tabAlways check your car’s owner’s manual or the fuel door to determine the recommended octane level for your vehicle. 
• Modern engines are adaptable: Opens in new tabMost contemporary vehicles have sophisticated computer systems that adjust to different fuel octanes. 
• High-performance engines: Opens in new tabVehicles with high-performance engines, turbochargers, or superchargers are more likely to be damaged by lower-octane fuel. 

What happens if I put 91 instead of 87 on Reddit?

Speaking in general terms: There is no benefit to running higher octane gas in an engine that does not require it or was not designed for it, but it is safe to do so.

Does 91 gas damage engines?

What happens if you use premium gas instead of regular? It’s unlikely that your car will be harmed if you use premium gas in a car that requires regular gas. But it’s a near-certainty that you won’t see an increase in performance and a fact that you’ll pay more for premium.