How does fuel octane work? What do the different ratings mean? There are two basic methods of measuring fuel octane: the RON (Research Octane Number) method and
the MON (Motor Octane Number) method. The RON measurement is made on an engine working at a relatively
light load, and the MON measurement is made on a heavily-loaded engine. There is very difficult mathematical
formula that relates the two measurements, but basically the difference is usually about 10 -- fuel with a
MON rating of 82 would have a RON rating of about 92.
Historically, the RON measurement was used as a standard. However, in the US, the octane measure posted on
gas pumps is the average of the RON and MON measurements, sometimes noted as '(R+M)/2' or AKI (Anti Knock Index).