My understanding of the subject is as follows.
USA 91 has a higher RON value than 91RON as used in New Zealand and Europe.
This is because US method is MON plus RON divided by 2.
Since the  MON rating for the same fuel is s always lower than the RON
rating, the result from dividing MON and RON by two will always give a
result lower than the straight RON number.
RON = research Octane Number, measured in the lab using a single cylinder
engine at constant RPM.
MON is motor octane number, multi cylinder engine tested at varying
revs.(i.e. real life)
Regards
John.
(PS. If I am wrong please feel free to educate me!)
----- Or
iginal Message -----

> > MArk, I believe that our octane is actually has more "Umph".  I believe
> > that the average method lessens the rating.
> > Anyone out there who can clarify?
> SNIP SNIP SNIP SNIP
>
> The European method, or the German method at least, uses the RON octane
> rating.  We, Yankees with big nukes, use a different method of calculating
> octane and the name escapes me at the moment.  91 RON octane is about 87
of
> ours.  If I am wrong I'm sure someone will correct me.  It never fails.  I
> still have the nukes, so watch out.