What “octane” means in petrol

Octane is a measure of a petrol’s resistance to engine knock (also called detonation); higher octane fuels are better at preventing knock in high-compression or turbocharged engines, but they don’t inherently contain more energy or make more power in engines that don’t require them. In practical terms, using the octane grade your vehicle specifies helps the engine run as designed, while higher grades are mainly beneficial if the engine is knock‑limited or tuned for them.

Why octane matters

In spark‑ignition engines, the fuel–air mix should ignite only when the spark plug fires. If it auto‑ignites early under pressure and heat, the resulting “knock” produces pressure spikes that can reduce performance, trigger the engine computer to retard timing (losing power and efficiency), and in severe or prolonged cases damage pistons or valves. A higher octane rating means the fuel is more knock‑resistant, allowing engines with higher compression ratios, turbocharging/supercharging, or aggressive ignition timing to operate safely and efficiently.

How octane is measured

Octane rating compares a petrol’s knock resistance to standard reference fuels in a controlled test engine. By definition, isooctane is rated 100 (very knock‑resistant) and n‑heptane 0 (knock‑prone). Real fuels are blended to match the knock behavior of those references under standardized conditions, yielding different rating scales that drivers see on pumps.

The following list explains the main octane rating systems you’ll encounter and how they appear at the pump in different regions.

• RON (Research Octane Number): Measured under lighter load and lower temperature; commonly displayed in the UK, EU, Australia, and many other markets. This is the figure behind labels like 95 or 98.
• MON (Motor Octane Number): Measured under heavier load and higher temperature; typically lower than RON for the same fuel.
• AKI (Anti‑Knock Index, shown as (R+M)/2): The average of RON and MON; used on pump labels in the United States and Canada. For example, US 87 AKI roughly corresponds to about 91–92 RON.

In short, RON is the headline number in most of the world, while North American pumps show AKI, which is lower for the same fuel because it averages RON and MON.

Typical pump grades by region

The following list gives common consumer grades, noting that offerings can vary by brand and locality.

• UK/EU (RON): Regular E10 is typically 95 RON; premium/super grades are usually 97–99 RON.
• United States (AKI): Regular 87, mid‑grade 89, premium 91–93 AKI. Some high‑altitude areas may sell 85 AKI regular, though many automakers advise 87 AKI minimum.
• Canada (AKI): Similar to US—87 regular, 89 mid‑grade, 91–94 premium, depending on province and brand.
• Australia (RON): Regular 91, premium 95, and premium 98 RON; E10 blends are common in some states.
• Japan (RON): “Regular” ~90 RON; “High‑octane” ≥96 RON, with 98–100 RON widely marketed.

These figures help translate labels across markets: a car calling for 95 RON in Europe will typically run on 91–92 AKI in North America.

Using higher or lower octane than recommended

Automakers specify a minimum octane for each engine. What happens if you go below or above it?

• Below the recommended octane: Modern engines will usually detect knock and pull ignition timing or boost to protect themselves, resulting in reduced power and efficiency; persistent heavy knock can cause damage, especially in performance engines or under high load/towing.
• At the recommended octane: The engine can run its intended timing and boost maps, delivering rated performance and efficiency.
• Above the recommended octane: There’s no extra energy in the fuel; most engines not knock‑limited will see little to no benefit. Some turbo/high‑compression engines can take advantage of extra knock margin to add timing/boost for modest gains, particularly in hot weather or under heavy load.

As a rule, match the owner’s manual. Use higher octane only if the manufacturer allows it for improved performance, or if you hear knock, tow heavy loads, drive in extreme heat, or have an engine tune that specifies it.

Common misconceptions

Octane is widely misunderstood. The following points separate myth from fact.

• Octane is not “power content”: It measures knock resistance, not energy. In fact, high‑octane blends or ethanol‑rich fuels can have slightly lower energy per litre yet enable more aggressive engine operation in engines designed for them.
• “Premium” doesn’t mean cleaner by default: Detergent additives that clean injectors/valves are separate from octane. Many fuels meet enhanced “Top Tier” detergent standards at all grades.
• Knock isn’t pre‑ignition: Knock is end‑gas auto‑ignition after the spark, while pre‑ignition is ignition before the spark (often due to hot spots). Both are harmful, but pre‑ignition can be more destructive and is less helped by octane alone.
• All engines don’t “run better” on higher octane: If an engine isn’t knock‑limited, higher octane yields no measurable benefit.

Understanding these distinctions helps you avoid paying more for fuel that your engine cannot use and focus instead on the correct grade and quality.

What influences octane—and today’s fuels

Fuel chemistry and regulations shape the octane you buy, and newer blends affect both knock resistance and drivability.

• Blend components: Branched hydrocarbons (like isooctane) and aromatics raise octane; straight‑chain alkanes lower it.
• Ethanol: A strong octane booster; E10 (up to 10% ethanol) is common in many markets. In the US, E15 (15% ethanol) offers higher octane than E10; beginning in 2025, summer sales are approved year‑round in several Midwest states, expanding availability.
• Volatility and aging: Storage and heat can evaporate lighter fractions, slightly reducing octane over time; fresh fuel matters for high‑performance engines.
• Altitude: Some high‑elevation regions sell lower AKI regular (e.g., 85 AKI) due to reduced cylinder pressures; check your vehicle’s guidance, as many manufacturers still recommend 87 AKI minimum.

These factors explain why the same nominal grade can behave differently by region, season, and brand, and why labels like E10 or E15 matter in addition to the octane number.

Choosing the right octane for your car

A few practical steps will help you select the correct petrol and avoid unnecessary cost.

• Follow the owner’s manual: Use the minimum octane specified (RON or AKI). If it says “required,” do not go lower; if it says “recommended,” higher octane may yield modest gains under certain conditions.
• Consider operating conditions: Hot weather, heavy loads, or mountain driving can push engines toward knock; higher octane can add margin.
• Pay attention to tuning: Performance tunes often require higher octane; using less can risk damage.
• Prioritize quality: Choose reputable brands and consider fuels that meet enhanced detergent standards for long‑term cleanliness, independent of octane.

Applying these guidelines ensures you get the performance and protection you paid for, without overspending on octane your engine can’t use.

Summary

Octane in petrol is a measure of knock resistance, not energy or “power.” It’s expressed as RON in many countries and as AKI ((R+M)/2) in the US and Canada. Use the grade your vehicle specifies: higher octane mainly benefits engines designed or tuned to exploit it, while lower octane risks knock, reduced performance, and potential damage. Ethanol content and regional rules influence the octane you see at the pump, with E10 common globally and broader US access to E15 in parts of the Midwest from 2025.

What does octane 87 mean?

87 octane gas is the standard, least expensive grade of gasoline at most U.S. gas stations and is suitable for most standard car engines. The number “87” is a rating of the fuel’s resistance to knocking or premature detonation inside the engine, which can be harmful. Using 87 octane fuel in a vehicle that requires it provides adequate performance and efficiency, while using higher octane fuel in such a vehicle is unnecessary and offers no benefit.
 
What Octane Means 

• Resistance to Knocking: Opens in new tabOctane measures a fuel’s stability and its ability to resist igniting before the spark plug fires in the engine’s cylinder.
• Anti-Knock Capability: Opens in new tabAn 87 octane fuel has the same anti-knocking capability as a mixture of 87% isooctane and 13% heptane, according to the Environmental and Energy Study Institute (EESI).
• High-Compression Engines: Opens in new tabHigher octane fuels allow for higher engine compression ratios, making them suitable for high-performance and turbocharged engines that require them.

Why 87 Octane is the Standard

• Most Cars: Opens in new tabThe vast majority of cars on the road are designed to run efficiently on 87 octane gasoline. 
• Cost-Effectiveness: Opens in new tab87 octane is the least expensive grade of gasoline available at most gas stations. 
• Performance and Efficiency: Opens in new tabUsing 87 octane fuel in a vehicle that requires it ensures efficient operation without wasting money on higher grades that offer no additional benefits. 

When to Use Higher Octane

• Check Your Manual: Always refer to your vehicle’s owner’s manual to determine the recommended octane level. 
• High-Performance Engines: Some sports cars and turbocharged engines are designed with higher compression ratios and require premium (higher octane) gasoline to prevent engine knock. 

Is octane the same as gasoline?

Octanes are a family of hydrocarbons that are typical components of gasoline. They are colorless liquids that boil around 125 °C (260 °F). One member of the octane family, 2,2,4-Trimethylpentane (iso-octane), is used as a reference standard to benchmark the tendency of gasoline or LPG fuels to resist self-ignition.

What does 95 or 98 octane petrol mean?

Gasoline with an octane rating of 98 is more resistant to self-ignition than gasoline with an octane rating of 95. 98-octane gasoline is generally thought to give more power than 95-octane gasoline.

Is higher octane petrol better?

One of the primary benefits of higher octane fuel is the reduction in engine knock. This can extend the life of the engine and prevent costly repairs. Knocking can cause significant damage over time, so preventing it can lead to long-term savings.