For more than 15 years I've been standing at the gas pump wondering, "what the heck is the (R+M)/2 method for determining the octane rating?"
I mean, is it really necessary to tell me the formula? Do different gas stations do it differently? Obviously, the sum of two numbers divided by 2 is an average, but what does it mean?
Here's an quote from csgnetwork
Gasoline pumps typically post octane numbers as an average of two
different values. Often you may see the octane rating quoted as
(R+M)/2. One value is the research octane number (RON), which is
determined with a test engine running at a low speed of 600 rpm. The
other value is the motor octane number (MON), which is determined with
a test engine running at a higher speed of 900 rpm. If, for example, a
gasoline has an RON of 98 and a MON of 90, then the posted octane
number would be the average of the two values or 94.
There you have it. So in a nutshell, it seems it's a test at slightly different RPMs.