Difference Between Fuel Ratings North American Region (NAR) uses the Anti-Knock Index (AKI), (RON+MON)/2, or (R+M)/2 when describing octane. In the Rest of the World (ROW), RON is used when describing octane. RON is the Research Octane Number and MON is the Motor Octane Number.   Anti-Knock Index (AKI) or (R+M)/2 In most countries in Europe (also in Australia, Pakistan and New Zealand) the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2.    Difference between RON, MON, and AKI Because of the 8 to 12 octane number difference between RON and MON, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel. This difference between RON and MON is known as the fuel's sensitivity and is not typically published for those countries that use the Anti-Knock Index labelling system.  As represented by their AKI and minimal octane ratings, there are currently three grades of traditional gasoline available in the U.S.: 87 (regular), 88 to 90 (mid-grade) and 91 to 94 (premium). For example, a 95 RON plus an 87 MON averages to a 91 AKI, which is the octane rating you would see displayed on a U.S. retail fuel site dispenser. The 95 RON and the 87 MON ratings can be derived from the exact same fuel. The test to determine the MON is more challenging than the RON test, and that’s why the RON test yields a higher number. Adding to the confusion: Europe uses the RON to label the fuel’s octane rating at the dispenser.  EU Ratings (RON) (MON) US Ratings (RON+MON)/2 92 85 88.5 95 87 91 96 88 92 98 90 94 100 92 95 105 95 100