What Really Happens When You Put 93-Octane Gas in a Car Rated for 87

In most cases, nothing bad happens—your car will run normally, and you’re unlikely to see any performance or fuel-economy benefit. Higher octane (93) simply resists knock better than regular (87); it doesn’t add power or act as a cleaner. For engines engineered for 87, using 93 is safe but usually a waste of money, with rare exceptions under very heavy loads or extreme heat.

Octane 101: What the Numbers Mean

Octane is a measure of a fuel’s resistance to engine knock (premature combustion), not its energy content or “quality.” In the U.S. and Canada, the number at the pump is AKI (Anti-Knock Index), or (R+M)/2. Regular is typically 87 AKI, mid-grade 89–90, and premium 91–93. Engines designed for 87 octane are calibrated to avoid knock on 87 under normal conditions.

How Modern Engines Handle Higher Octane

Most vehicles built in the last two decades use electronic engine control units (ECUs) with knock sensors. These systems continually adjust ignition timing to keep combustion stable and protect the engine. If you run 93 in a car designed for 87, the ECU generally won’t find extra timing “headroom” because the calibration isn’t knock-limited on regular fuel. The result: it behaves the same as on 87, with no meaningful bump in horsepower or miles per gallon.

Possible Niche Benefits

There are edge cases. In very hot weather, when towing, climbing long grades, or at high altitude, some engines might pull a bit less timing on higher octane, yielding slightly smoother operation. The difference is typically subtle and not cost-effective for everyday driving.

Common Myths vs. Reality

These are the most widespread misconceptions about premium fuel in regular-fuel cars and how they compare with what experts and automakers state.

• Myth: Premium has more energy and boosts power. Reality: Octane controls knock resistance, not energy content. Any power gain in an 87-rated engine is negligible.
• Myth: Premium cleans the engine. Reality: Detergent additives do the cleaning, and they’re mandated in all grades. “Top Tier” gasoline standards matter more than octane.
• Myth: Using 93 prolongs engine life in 87 cars. Reality: There’s no demonstrated longevity benefit; modern knock control already protects engines on the recommended fuel.
• Myth: Premium hurts emissions systems in regular-fuel cars. Reality: It doesn’t. Catalytic converters and O2 sensors are unaffected by higher octane.

In short, premium isn’t a performance booster, a cleaner, or a longevity hack for engines designed to run on regular—it’s just more knock-resistant fuel.

When Premium Actually Matters

Some vehicles, often turbocharged or high-compression models, specify “premium required” or “premium recommended.” That’s a different situation from an 87-rated car.

Here’s how those categories differ and why they matter.

• Premium required: Use 91/93 as specified. Running 87 can cause severe knock, reduced performance, potential engine damage, and may jeopardize warranty coverage.
• Premium recommended: The engine can safely run 87, but performance and efficiency may drop, and the engine may feel less responsive.
• Regular (87) specified: The engine is designed and tuned for 87. Using 93 generally brings no measurable benefit.

Matching the owner’s manual is the best way to get the performance, efficiency, and durability the engine was designed to deliver.

Potential Downsides of Using 93 in an 87 Car

There’s no mechanical harm, but there are practical drawbacks.

Consider these factors that might make premium unnecessary or even counterproductive to your budget.

• Cost: Premium often costs 10–20% more per gallon with little or no gain in economy or power.
• Ethanol content variability: Energy content can vary by ethanol blend (E10 vs. E0), not by octane. Any MPG differences typically stem from ethanol percentage, not the 93 rating.
• Availability quirks: In rare cases, pumps labeled E15 (15% ethanol) may be paired with higher octane; vehicles 2001 and newer are generally approved for E15, but always check your fuel door and owner’s manual.

For most drivers of 87-rated cars, the premium upcharge is money left on the table, unless a specific blend offers a non-octane advantage you actually need.

Mixing Fuels and “Accidental” Fill-Ups

Drivers sometimes worry after topping off with the “wrong” octane. The reality is straightforward.

Keep these practical points in mind if you mix 87 and 93 or fill up differently than usual.

• Octane averages out: A half tank of 87 and a half tank of 93 give you roughly 90 AKI. Your ECU adapts accordingly.
• No need to drain: For an 87-rated car, 93 won’t harm anything. Just drive normally and refuel as usual when needed.
• Watch ethanol labels: If you inadvertently chose E15 and your vehicle isn’t approved for it (most 2001+ cars are, but verify), return to your normal fuel at the next fill.

In everyday scenarios, an off-spec octane fill is not an emergency; modern fuel and engine controls are designed to handle it.

What You Should Do

To get the best balance of cost, performance, and engine health, follow these straightforward practices.

These guidelines reflect current industry recommendations from automakers and fuel quality standards.

• Use the octane your owner’s manual specifies (typically 87 for many mainstream cars).
• Prioritize Top Tier detergent gasoline, regardless of octane, to keep injectors and intake valves cleaner over time.
• Consider higher octane only for sustained heavy-load conditions (towing, extreme heat) if you notice audible knock or the manual suggests it.
• Don’t expect premium to fix performance issues—address maintenance items like spark plugs, air filters, and diagnostics if the car feels sluggish.

Following the manufacturer’s guidance and focusing on fuel quality, not just octane, pays the biggest dividends.

Bottom Line

Putting 93-octane gas in a car designed for 87 is safe and usually uneventful: no damage, no meaningful gains, just a higher fuel bill. Save premium for engines that require or clearly benefit from it, and emphasize reputable, Top Tier fuel and proper maintenance for long-term reliability.

Summary

Higher octane resists knock; it doesn’t inherently add power or cleanliness. In an 87-rated car, 93 octane won’t harm the engine but rarely improves performance or fuel economy. Stick to the manufacturer’s recommended octane, choose Top Tier fuel when possible, and reserve premium for engines that specify it or for unusual, high-load conditions where it demonstrably helps.

What happens if you accidentally mix 87 and 91 gas?

Accidentally mixing 87 (regular) and 91 (premium) octane gasoline is generally harmless for most modern vehicles because the octane levels will average out, creating a fuel with a mid-range octane rating. While engines have knock sensors to adjust ignition timing and prevent issues from lower octane fuel, it’s still best to avoid mixing fuel grades, especially in cars that require premium gas, to prevent potential long-term engine damage from detonation. However, as a one-time occurrence, the engine will likely be fine and may experience only slightly reduced power output.
 
What happens when you mix octane levels

• Octane averaging: When you mix 87 and 91 octane fuel, the resulting fuel will have an octane rating somewhere in the middle, such as 89 or 90, depending on the ratio. 
• Engine knock: High-octane fuel is less likely to ignite prematurely under pressure, which is known as “engine knock” or “detonation”. 
• Knock sensors: Modern vehicles are equipped with knock sensors that can detect this phenomenon and adjust the ignition timing to prevent damage. 
• Potential for reduced power: While your car will likely adjust, you might notice a slight decrease in engine performance or power, although this is often not noticeable to most drivers. 

When to be cautious

• High-compression engines: In engines that require premium fuel, especially high-performance or older vehicles that may not have advanced knock sensors, using lower octane fuel could lead to detonation and damage. 
• Frequent mixing: This advice applies to a one-time, accidental mixing of fuels. Consistently using a lower octane fuel than your car requires, even if it’s a blend, is not recommended and can cause long-term damage. 

What to do if you’re concerned 

• Avoid aggressive driving: Opens in new tabIf you’ve mixed fuel and are concerned, avoid aggressive driving, such as hard acceleration, to minimize the risk of knocking.
• Fill with the correct octane soon: Opens in new tabTop off your tank with the recommended fuel octane to help bring the average octane level back up to the correct specification.
• Listen to your engine: Opens in new tabMonitor for any unusual engine noises, especially knocking or pinging, and consult a mechanic if symptoms persist.

Is it bad to put 93 in a car that takes 87?

No, it is not bad to put 93 octane gasoline in a car that typically takes 87 octane; it will not cause damage to the engine and is a safe alternative if higher-octane fuel is the only option available. The main effect will be wasted money, as the car’s engine cannot take advantage of the higher octane rating. Some modern engines may experience minor improvements in performance or fuel economy, but for most vehicles, there will be no noticeable difference other than the cost. 
What to expect:

• No engine damage: Your car is designed to be safe with 87 octane fuel, so a higher octane fuel will not harm it. 
• Wasted money: You are essentially paying more for a fuel that your car’s engine is not designed to benefit from. 
• No performance gain: Your engine will not suddenly gain horsepower or improve significantly, as it is not tuned to utilize the higher octane fuel. 
• Potential for slight changes (modern cars): Some modern vehicles with advanced computer systems and sensors, like knock sensors, can adapt to higher octane fuels, potentially leading to minor improvements in responsiveness. 

When it is important:

• Use recommended octane: Opens in new tabAlways check your car’s owner’s manual to confirm the recommended octane for your specific vehicle. 
• Don’t use low octane for high-requirement engines: Opens in new tabIf your car requires premium gasoline due to a high-compression or turbo-charged engine, using lower-octane fuel can cause engine knocking, reduced performance, and potential long-term damage. 

Can you put 93 in an 87 car on Reddit?

If the car states to use 87 octane and you run 93, the car will run normally and there is no issue. If the car states to use 93 octane and you run 87, you risk potential engine damage.

Can you mix 87-89 and 93 gas?

You can absolutely mix octanes, and they mix linearly.