Octane rating: RON, MON and AKI explained
An octane rating measures how well petrol resists knock, an uncontrolled ignition in the engine. A higher number resists it better. It is mostly about how much energy the fuel holds.
In this entry
What an octane rating measures — petrol grades and knock resistance
An octane rating describes how well petrol resists knock. Knock, sometimes heard as pinging, happens when part of the fuel and air mixture in a cylinder ignites by itself instead of burning smoothly after the spark. Over time it can damage an engine.
A higher number means the fuel tolerates more pressure and heat before it ignites. It does not mean the fuel holds more energy. That is why a car built for 95 does not go further on 98, and why engines with high compression or turbochargers often ask for higher grades.
Petrol grading and petrol grades are based entirely on octane rating. Standard petrol grades sold at the pump — Regular (87 AKI in North America), 95 RON in Europe, 91 RON in some Asian markets, and Premium at 98–100 RON — all represent different points on the same knock-resistance scale. The grade the manufacturer specifies for your engine is the minimum you should use; higher grades are safe but seldom beneficial unless the engine management system is tuned to exploit them.
Octane and cetane are the two parallel quality measures for the two mainstream fuel types. Octane rating applies to petrol (gasoline); cetane number is the equivalent for diesel. The difference between octane and cetane is fundamental: petrol engines use a spark plug to ignite fuel that must resist self-ignition, so a higher octane number is better; diesel engines rely on the heat of compression to ignite the fuel, so they need a fuel that ignites easily — which means a higher cetane number is better too, but for the opposite reason. In a cetane vs octane comparison: high-octane petrol would be a poor diesel fuel because it resists the compression ignition that diesel engines depend on, and high-cetane diesel would be a poor petrol fuel because it would knock badly in a petrol engine. See the cetane number glossary entry for the diesel side of this comparison.
RON, MON and AKI: three ways to write the number
The same fuel can carry different numbers depending on how it is tested. Research Octane Number (RON) is measured under milder engine conditions. Motor Octane Number (MON) uses harsher conditions, so it is usually several points lower. Some countries show one of them, and others show an average.
The reason US numbers look lower than European ones for the same fuel.
How the numbers line up
Because AKI averages in the lower MON figure, a US number looks smaller than the European number for the same fuel. As a rough guide, 87 AKI is about 91 to 92 RON, 89 AKI is about 93 to 94, 91 AKI is about 95 to 96 and 93 AKI is about 97 to 98. Exact values vary by fuel batch, so treat the chart as an approximation.
Approximate marks on the RON scale. Blue blocks are US grades, dots are common RON grades.
Does higher octane mean more power?
Only if the engine is built to use it. A car designed for 95 RON gets no extra power from 98 in normal driving. What octane does is let an engine with high compression or turbocharging run its ignition timing more aggressively without knocking.
The practical rule is to use the grade the manufacturer specifies. Going higher is safe in almost every petrol car, but it usually costs more with no benefit. Going lower can cause knock, and many modern engines respond by reducing power to protect themselves.
Altitude, ethanol and diesel
Thinner air at high altitude reduces knock, which is why some mountain regions in the western United States sell an 85 AKI regular grade.
Ethanol raises octane. E85 is typically in the range of 100 to 105 RON, but it holds less energy per litre, so a car burns more of it per kilometre. Compare it by cost per kilometre, not cost per litre.
Diesel engines do not use octane at all. They rely on ignition by compression, so diesel quality is described by the cetane number instead. The European diesel standard EN 590 sets a minimum cetane number of 51.
How grades work on FuelTheGuide
Countries sell different grades, so we show each fuel the way it is labelled locally: 92, 95, 98 or 100 in most of the world, and Regular 87, Midgrade 89 and Premium in the United States.
For side-by-side comparisons we group grades into standard and premium petrol, using each country's usual grade as the standard. These groups are approximately US Regular (2 to 3 AKI points below Europe's 95 on the RON scale), so a price gap between them is not perfectly like for like.
Questions and answers
What is the difference between octane and cetane?
Octane and cetane are both fuel quality ratings, but they measure opposite things. Octane rating applies to petrol: it measures how well petrol resists knock (premature self-ignition in the combustion chamber). A higher octane number is better for petrol because petrol engines use a spark plug and need the fuel to wait for that spark. Cetane number applies to diesel: it measures how readily diesel ignites when compressed. A higher cetane number is better for diesel because diesel engines rely on compression heat to ignite the fuel without a spark. In a cetane vs octane comparison: a high-octane petrol would resist the compression ignition that diesel engines need; a high-cetane diesel would self-ignite too early in a petrol engine, causing knock. See the cetane number entry for the diesel side.
What petrol grades are sold at the pump?
Petrol grades vary by country but all represent points on the octane scale. In the US and Canada, petrol grading uses AKI: Regular 87, Midgrade 89, and Premium 91–93. In Europe, the UK, Australia and most of Asia, the RON scale is used: typically 91, 95 and 98 RON, with some markets offering 100 RON. Because AKI averages in the lower MON figure, US grades look about 4–5 points lower than their European RON equivalents for the same fuel — US Regular 87 AKI is approximately 91–92 RON.
Is 95 octane the same as 91 octane?
Not on the same scale. 95 is usually a RON figure and 91 is an AKI figure. As a rough guide, 87 AKI is about 91 to 92 RON, 89 AKI is about 93 to 94, so the fuel can be almost the same fuel.
Can I put higher octane petrol in my car?
Yes, but it usually costs more with no benefit. Going lower can cause knock, and many modern engines respond by reducing power to protect themselves.
What happens if I use lower octane than recommended?
Modern engines with knock sensors will retard the ignition timing to prevent knock, which reduces power and fuel economy. In older engines without knock sensors, using lower octane fuel than required can cause engine damage over time. The US DOE fueleconomy.gov explains octane requirements and knock in plain language.
Do diesel engines have an octane rating?
Diesel engines do not use octane at all. They rely on ignition by compression, so diesel quality is described by the cetane number instead. The European fuel quality directive EN 590 sets a minimum cetane number of 51 for standard diesel across the EU.
RON vs AKI: why the numbers look different worldwide
Understanding the difference between RON vs AKI is essential when comparing fuel across borders. A driver filling up with what is labelled 95 RON petrol in London is getting roughly the same knock resistance as a driver buying 91 AKI in New York, yet the numbers on the pump look quite different. The confusion arises because RON (Research Octane Number) is tested under gentler engine conditions, while AKI averages RON with the harsher MON test, pulling the displayed number down by four to six points. The U.S. Energy Information Administration provides a detailed breakdown of how octane is measured and why AKI is the standard in the United States.
If you are travelling or importing a vehicle, always convert between scales before choosing a grade. What is 95 RON petrol in practice? It is the standard unleaded grade across Europe, the UK, Australia and most of Asia — equivalent to roughly 90-91 AKI. Premium grades such as 98 or 100 RON map to approximately 93 AKI. The UK government's fuel guidance confirms that 95 RON E10 is the default pump grade nationwide, while the European Commission's energy market pages track how fuel quality standards are harmonised across EU member states.
More questions
What is 95 RON petrol and is it the same as US regular?
95 RON petrol is the standard unleaded grade in Europe, the UK and most of Asia. It is not the same as US Regular on a like-for-like basis: US Regular is 87 AKI, which equates to roughly 91-92 RON. So 95 RON petrol actually sits between US Midgrade (89 AKI) and Premium (91-93 AKI). The difference matters when an owner's manual specifies a minimum octane — always check whether the figure is quoted in RON or AKI before choosing your grade.
Sources and further reading
- Gasoline explained: octane in depth eia.gov
- EN 590 (petrol) and EN 590 (diesel) fuel standards European Commission
- Fuel prices and grade lists openvan.camp, open dataset, CC BY 4.0