system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what's the difference between 87, 89, and 93 octane?

And is one better than the other for my car?
gpt
In a petrol (gasoline) engine the fuel is supposed to ignite when the spark happens. But it's hot in the cylinder and that might make the fuel ignite before the spark happens, which is known as engine pre-ignition, and that makes the fuel burn incorrectly causing engine 'knocking' or 'pinging'. At best knocking makes a noise and reduces power. At worst it can destroy the engine.

The octane rating of a fuel measures its resistance to pre-ignition. It's a measurement, made using a special test engine, and does not necessarily relate to the amount of actual octane chemical in the fuel.

Your car's engine will have been designed for a certain octane rating. Use that if you can. Using lower octane in a modern car won't normally do any harm because the engine computer adjusts but it must do so by limiting the engine's power. So if your car should have 93 but the gas station only has 89 don't worry too much. Using lower octane in an older car may cause knocking. Using higher octane gives no benefit and wastes money.

On a final note, Europe measures octane rating a bit differently to the USA. European 98 is about the same as US 93. So if it ever seems like European fuels have crazy high octane, they don't, it's just the units measuring it.