Gas Pump Trivia

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. 

Sources:
http://www.csgnetwork.com/octaneratecalc.html
http://www.havoline.com/carcare/glossary/r.html
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