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
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.
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
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.
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