When a mixture of gasoline (petrol) and air is compressed inside an engine cylinder it heats up. If the compression of the engine is high enough, and if the fuel is able to ignite easily enough, the air-fuel mixture may spontaneously ignite before the spark plug is fired at the optimum ignition moment. This is called 'premature detonation', better known as 'knocking'.
Gasoline fuel can be modified in manufacture, or through the addition of additives, so that it is less prone to spontaneously ignite. This ignition characteristic is known as its 'Octane Rating'. The less easily the fuel ignites, the higher the octane rating. Higher compression engines are more susceptible to engine knock, so they use fuels with a higher octane rating, that is, fuels that ignite less readily.
There are two different methods used to measure the octane rating of a fuel, which result in the Research Octane Number (RON), and the Motor Octane Number (MON). The MON number is usually about 10 points lower than the RON number. Both are measurements of a fuel's resistance to knock, or premature detonation, but the MON is a better measure of how the fuel behaves when under load.
In most countries, including the whole of Europe and much of the rest of the world, the RON rating is the one that is usually displayed on the pump at filling stations. In the USA and Canada, and some other countries, the displayed fuel rating is an average of the RON and the MON rating. Consequently, whatever the rating may be called at the pump, the rating number for identical fuels will on average be about 5 points higher in Europe than they will appear to be in the USA.
There is a popular belief that higher octane rated fuels will give improved performance in cars that are designed to run on lower octane rated fuels. Although some premium fuels also have higher energy ratings, this is largely a myth. Higher powered engines usually have a higher compression ratio, so require the generally more expensive higher octane fuels to prevent premature detonation. Higher octane does not in itself mean higher energy output, so a fuel designed for a high compression engine will not necessarily deliver any more power in a lower compression engine. Engines perform best when used with the fuel that has the engine manufacturer's recommended octane rating.