Just a note to remember that octane is often quoted using different measurement systems in different parts of the world.
For example, “petrol” in the UK/Europe is sold using a RON (Research Octane Number) measure of octane that Barry uses above, while “gasoline” in the US/Canada is sold using an average of the RON and the MON (Motor Octane Number). Bob Roller above is using (RON+MON)/2. RON is a typically 8 to 10 numerical points higher than MON, therefore the average (RON+MON)/2 is 4 to 5 numerical points higher than MON.
There’s lots of reasons why one system of measurement might be preferred over another, but the difference between the typical UK/Europe and US/Canada methods of presentation means that the typical US/Canada 87 octane “regular” gasoline is equivalent to between 91 and 92-octane in the UK/Europe. In much of the UK/Europe, “regular” petrol is 95-octane, which is about the same as “mid-grade” 90ish-octane gasoline in the US/Canada.
Mark