Yes, they're calculated differently. This covers it pretty well: https://en.wikipedia.org/wiki/Octane_rating#Measurement_methods https://en.wikipedia.org/wiki/

Yes, they're calculated differently. This covers it pretty well: https://en.wikipedia.org/wiki/Octane_rating#Measurement_methods https://en.wikipedia.org/wiki/Octane_rating#Measurement_meth... TLDR, the US and Canada use "(R+M)/2" (also called Anti-Knock Index), and much of the rest of the world uses RON (Research Octane Number). If you zoom in closely on the first image in my comment above, you can see that the sticker has both. It says "MIN.(R+M)/2 87 Regular" and "MIN.RON 91 Normale". --- Tangentially, you will see lower-octane fuel in specific parts of the US. Denver, Colorado is nicknamed "the mile-high city" because its elevation is about 1 mile (1.6 km) above sea level. In many parts of the US, you will see octane ratings of around 87, 89, and 93 at the pump, but in Denver you might see 85, 87, and 91. The thinner air reduces the need for high octane. I suppose something similar must happen in Europe, but it may not be as common or well-known. From what I can tell, non of Europe's large cities are at that high of an elevation.