Measurement methods
The most common type of octane rating worldwide is the
Research Octane Number (
RON). RON is determined by running the fuel through a specific test
engine with a variable
compression ratio under controlled conditions, and comparing these results with those for mixtures of isooctane and n-heptane.
There is another type of octane rating, called
Motor Octane Number (
MON) or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable
ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.
In most countries (including all of
Europe and
Australia) the "headline" octane that would be shown on the pump is the RON, but in the
United States and some other countries the headline number is the average of the RON and the MON, sometimes called the
Anti-Knock Index (
AKI),
Road Octane Number (
RdON),
Pump Octane Number (
PON), or
(R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the
US and
Canada, would be 91-95 (regular) in Europe.
