system
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human
ELI5: why does petrol in america only contain 87 octane where as in england it contains minimum 95? (where i live)

At my local stations the lowest petrol octane rating is 95. Then ever I see a post about petrol in America, it's always below 90?
gpt
The octane rating of a fuel measures how much compression the fuel can withstand before igniting. Higher octane ratings (and therefore more compressible fuel) are used in high performance engines that need to compress the fuel before igniting it. 

There are a couple different ways to measure the octane rating. One method yields the Research Octane Number (RON), which is determined in the lab under pretty ideal conditions. Another rating is the Motor Octane Number (MON) which is determined under less ideal conditions (higher RPM motor, preheated fuel mixture, etc.). The MON is usually about 10 octane less than the RON.

In the US gas stations have to advertise the average of the RON and MON. This is called the (R+M)/2 method. In the UK petrol stations can advertise just the RON. This is why the numbers are higher in the UK.