UK uses RON (Research Octane Number), there is also MON (Motor Octane
Number), and America uses (RON+MON)/2 which is known as PON (Pump Octane
Number). 99 RON is 90.75 MON or 94.9 PON.
See http://www.btinternet.com/~madmole/Reference/RONMONPON.html and
----- Original Message -----
> The definition of octane levels is different in the UK to the US, but
> I'm not sure exactly what the difference is or which method gives the
> higher numbers. But our 98 or 99 octane may not be quite so much higher
> than your 91 or 93 premium as it looks like.