Europe uses only the research octane number (RON), while in the US, we use the average between the research octane number (RON) and the motors octane number (MON). That's why on our pumps you see (something like), R+M/2.
Research number is derived from iso-octane and motor is derived from n-heptane, and research is the higher of the two numbers. RON is figured on a test that simulates fuel performance under low operating speeds (600 RPM I believe), while MON simulates high speed (9-1200 RPM). It is primarily a tool to compare fuel at a pump from different distributors.
In Europe they only use the research number. As a general rule of thumb, subtract 4 from the European octane rating and you have what the R+M/2 number would be in the US.
(NOTE: I say European, but it is primarily the number used in all countries but the US. Similar to the metric system that we have failed to adopt officially.)
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