[Jeff Niceswanger]

What's the RM rating for av gas? Is the rating your seeing Research Octane?
Research Octane Number (RON) The RON test measures a fuel's anti-knock performance under relatively mild engine conditions, such as those experienced during low-speed driving and acceleration. Test conditions: The fuel is tested in a variable compression ratio engine at a low engine speed of 600 rpm.Significance: RON is the most common octane rating used around the world, particularly in Europe, Australia, and many other countries. Motor Octane Number (MON) The MON test measures a fuel's resistance to knocking under more severe, high-load engine conditions, such as high speed or highway driving. Test conditions: The fuel is tested in the same engine used for the RON test but under harsher conditions, including a higher engine speed of 900 rpm and a preheated fuel mixture. Significance: The MON is typically 8 to 12 points lower than the RON for the same gasoline because the test conditions are more demanding. The \((R+M)/2\) Index For gasoline sold in the United States and Canada, the advertised octane rating is the average of the RON and MON values. Calculation: AKI = \((RON+MON)/2\).Example: A fuel with a RON of 91 and a MON of 83 would have an AKI of 87, which is what's displayed on the pump. International comparison: As a result of this averaging, the same fuel will have a lower octane number on a North American pump than in countries that use the RON standard. For example, 87 AKI fuel in the U.S. is equivalent to 91 RON fuel in other parts of the world.

I noticed the Turbo Blue at a local gas station pump is 100 octane. Its 105 Research and 96 Motor Octane. (Just look at the yellow R+M sticker on the pump face).
100 plus 96 = 201 divided by 2 = 100.5. So, its advertised to us racers at 105 but if tested by street gas ratings is only 100.
In the early 1970s, the Sunoco 260 premium fuel had an octane rating of 97.5. It was sold at "Custom-Blended" pumps, which allowed customers to choose from a range of octane levels. It was rated with the higher "Research" octane as the RM/2 method had not been implemented back then. Based on the difference between 1970s and modern octane rating methods, the 1970s version of Sunoco 260, which had a Research Octane Number (RON) of 97.5, would have likely been rated between 93 and 95 using the modern (R+M)/2 method.