what is R+m/2 formula for octane, what is the "R", what is the "M"

Hello lacrossecane and thank you for your question.

The R means "Research Octane Number"
The M means "Motor Octane Number"

"(R+M)/2
Research Octane Number plus Motor Octane Number, divided by 2. Used
now as general measure of road octanes of gasoline."
http://www.havoline.com/carcare/glossary/r.html

"Research Method- Research Octane Number (RON) 
A test for determining the knock rating, in terms of ASTM Research
octane numbers, of fuels for use in spark-ignition engines. The
knocking tendency of the fuel is compared with those for blends of
reference fuels of known octane number when run in the ASTM-CRF engine
at 600 rpm under standard operating conditions (ASTM Methods D908 and
D 1656)."
http://www.chevrontexacoursa.com/glossary/r.html

"Motor Method- Motor Octane Number (MON) 
A test for determining the knock rating, in terms of ASTM Motor Octane
Numbers, of fuels for use in spark-ignition engines. The knocking
tendency of the fuel is compared with those for blends of reference
fuels of known octane number when run in the ASTM-CFR engine at 900
rpm, under standard operating conditions as prescribed in ASTM Method
D 357."
http://www.chevrontexacoursa.com/glossary/m.html 

Thank you very much for your question.

Very best regards

THX1138

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