Originally Posted By: Defiant
Yes- that R + M / 2 gibberish you see below the octane number on that big yellow square on the gas pump means, "Research octane" -derived from chemical analysis, plus "Motor octane" -the point at which one of Ricardo's engine designs knocks to a predetermined level as compression is increased, added up and divided by two to come to a determination of a gasoline's resistance to knock (pre-ignition, not "ping").
Not quite. Research Octane Number (RON) is determined by the same engine as Motor Octane Number (MON), just under different conditions. From memory RON is run with variable ignition timing whereas MON runs with fixed ignition timing, making it a more severe test (hence why MON is always lower than RON). There are also some other differences (I'm stretching my memory here, but maybe something to do with inlet air temperature and some other parameters).
The maths after that is, as you say, the average of the RON and MON, sometimes called the Anti-Knock Index (AKI). Over here (and most places outside of there) we use only RON values on pump fuel.