In answer to the question as to whether the same or different methods are used to measure octane in the US and Europe, I've posted (below) what Wikipedia (which is not necessarily the most definative source) has to say about the methods of rating ... and about Regional differences ...
================================================== ===================================
Measurement methods
The most common type of octane rating worldwide is the
Research Octane Number (
RON). RON is determined by running the fuel in a test
engine with a variable
compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane 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 rating, shown on the pump, is the RON, but in the
United States,
Canada 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, the octane shown in the United States is 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the
US and
Canada, is 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90-91 US (R+M)/2, and some even deliver 98 (RON) or 100 (RON).
It is possible for a fuel to have a RON greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels,
AvGas,
LPG, and
alcohol fuels such as
methanol or
ethanol can have octane ratings of 110 or significantly higher - ethanol's RON is 129 (MON 102, AKI 116). Typical "octane booster"
gasoline additives include
tetra-ethyl lead,
MTBE and
toluene. Tetra-ethyl lead (the additive used in leaded gasoline) is easily decomposed to its component
radicals, which react with the radicals from the fuel and oxygen that start the combustion, thereby delaying ignition, leading to an increased octane number.
Regional variations
Octane ratings can vary greatly from region to region. For example, the minimum octane rating available in much of the United States is 87 AKI and the highest is 93. However this does not mean that the gas is different.
In the Rocky Mountain (high altitude) states, 85 octane is the minimum octane and 91 is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical combustion engine draws in less air per cycle due to the reduced density of the atmosphere. This directly translates to reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a car with a carburetor that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI is available
[1]. In parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri)
ethanol based
E-85 fuel with 105 AKI is available
[2].
California fuel stations will offer 87, 89, and 91 octane fuels, and at some stations, 100 or higher octane, sold as racing fuel. Until Summer 2001 before the phase-out of methyl
tert-butyl ether aka
MTBE as an
octane enhancer additive, 92 octane was offered in lieu of 91.
Generally, octane ratings are higher in
Europe than they are in
North America and most other parts of the world. This is especially true when comparing the lowest available octane level in each country. In many parts of Europe, 95 RON (90-91 AKI) is the minimum available standard, with 97/98 being higher specification (being called
Super Unleaded). In
Germany, big suppliers like
Shell or
Aral offer 100 octane gasoline (Shell V-Power, Aral Ultimate) at almost every gas station. In
Australia, "regular" unleaded fuel is RON 91, "premium" unleaded with RON 95 is widely available, and RON 98 fuel is also reasonably common. Shell Used to sell RON 100 petrol from a small number of service stations, most of which are located in capital cities (stopped in august 2008). In Malaysia, the "regular" unleaded fuel is RON 92, "premium" fuel is rated at RON 97 and Shell's V-Power at RON 99. In other countries "regular" unleaded gasoline, when available, is sometimes as low as 85 RON (still with the more regular fuel, 95, and premium, around 98, available). In
Russia and
CIS countries 80 RON (76 MON) is the minimum available, the standard is 92 RON, however, the most used type is 95 RON.
This higher rating seen in Europe is an artifact of a different underlying measuring procedure. 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, Canada 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 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90–91 US (R+M)/2, and deliver 98 (RON), 99 or 100 (RON) labeled as
Super Unleaded.
In the
United Kingdom, 'regular' petrol has an octane rating of 95 RON, with 97 RON fuel being widely available.
Tesco and
Shell both offer 99 RON fuel.
BP is currently trialling the public selling of the super-high
octane petrol BP Ultimate Unleaded 102, which as the name suggests, has an octane rating of RON 102. Although BP Ultimate Unleaded (with an octane rating of RON 97) and BP Ultimate Diesel are both widely available throughout the UK, BP Ultimate Unleaded 102 is (as of October 2007) only available throughout the UK in 10 filling stations.