Originally Posted by Polwart
Except that in the USA the Octane rating used at the pump is not the RON number as it is in the UK. It is measured differently. UK standard road petrol (95 RON) would probably be sold in the US as 91 Octane (the exact number depends on the blend - they are not directly convertable like say US gals to UK gals). Similarly the standard road gasoline in the US which is sold as 87 octane would be sold in the Europe as something like 92 RON.
That's right; in the US, the quoted octane is RON + MON / 2, where MON is a tougher, more real life test and is nearly always lower than RON.
Therefore, US quoted octane figures at the pump will be lower than ours for the same fuel.
Nonetheless, the overall gasoline pool in the US has a lower octane than that of Europe, with the vast majority sold being 'regular' 87, (about 91 RON in Europe). This is doubtless due to the long US history of large, relatively 'basic' engines, eg. big cc V8's, etc. that don't make that much power for their size.