why it’s common to have 87-octane gasoline in the US but it’s almost always 95-octane in Europe?
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Chemical engineer here working in refining and specifically in gasoline production.
Some of the other comments about Europe using Ron vs. Mon are correct, however, there is additional info:
European vehicles trend toward smaller displacement, higher compression ratios in their vehicles. Why? Because high compression ratios make a more efficient vehicle. What is a compression ratio? It is how much pressure you apply to the air/fuel mixture before setting off the spark.
Ok, so why is octane important here? All fluids heat up when compressed, if you heat up an air/fuel mixture beyond it’s “flash point”, it will auto ignite causing an explosion without the piston being in the right position for said explosion. This is what’s called “knock” and is very hard on your engine.
So, higher octane means you can compress the air/fuel mixture more without knock, which leads to higher engine efficiency
Edit: I want to make a comment about a common misconception here. A higher octane does not mean there is more energy per gallon of fuel in any meaningful way. The engine must be designed to fully utilize the properties of the higher octane fuel. If you just shove some high octane in your 87min Honda Accord, you won’t get any better fuel economy. In some cases, because of the fuels resistance to ignition, you can actually end up causing some carbon deposits to occur in the engine.
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