The whole octane discussion gets very confusing when you start comparing the US against the RoW. The standards listed on our pumps are generally based on AKI as I recall. RON is in Europe.
The formula that vracer mentioned (RON + MON / 2) is also visible on fuel pumps in CT. Rather than bungle up the discussion here's what Wikipedia says about the comparison:
Wikipedia said:
In the US, octane ratings in unleaded fuels can vary between 86 and 87 AKI (91-92 RON) for regular, through 89-90 AKI (94-95 RON) for mid-grade (European premium), up to 90-94 AKI (95-99 RON) for premium (European super).
In CT, regular is typically listed as 87, midgrade/special is 89 and premium is generally 91 (maaaybe 93 if you can find it). Mobil's premium used to be 93, for example, but I think local regulations have altered the mix taking it down to 91. Doesn't Sunoco sell a "94" in some states? I believe those are considered "super" here.
-Eric