Thanks! Read this in an article though, ?In Singapore, the number at petrol pumps is the RON. However, in the United States, it usually indicates 'mean' octane number, that is, (RON + MON)/2, or the average of both the RON and the MON. This is why there is an apparent discrepancy between octane values of petrol in the US and the rest of the world. For example, RON95 unleaded in Singapore is 95 octane but equivalent to American (RON+MON)/2 89 octane.
In the US, low-altitude petrol stations typically sell three grades of petrol with octane ratings of 87, 89 and 91. High-altitude stations typically also sell three grades, but with lower values - 85, 87 and 89. This is because, for every 300m increase in altitude, the RON value goes down by 0.5.?