Octane rating is the resistance of a fuel to knocking.

Some petrol engines can adjust the ignition timing to take advantage of higher octane fuels, and to prevent engine damage if low octane fuel is used. But in most petrol engines there is no advantage in using a higher octane fuel than the maker recommends.

There are two different methods of measuring octane number, and they give different results. The Research Octane Number (RON) is the more widely quoted, and is higher than the Motor Octane Number (MON).

European petrol is available in several grades, with different octane numbers:

  • 91RON Unleaded (not sold in UK)
  • 95RON Premium Unleaded
  • 97RON Super-Unleaded
  • 99RON eg Shell V-Power