system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the actual difference between 87, 89, and 91 octane gas and what effect does it have on my vehicle?

Always wanted to know.
gpt
Octane ratings are essentially the resistance of a particular fuel to pre-ignition. This used to have some direct correlation to percentage of octane in the fuel, but that's not so anymore.

If your engine has a high compression ratio, uses a turbocharger, or is otherwise "high-performance," you will most likely be required to use 91 octane or above. If you have a fairly ordinary engine, 87 octane is probably just fine. You should read the manual for your car/engine and follow its advice.

If you run high-octane fuel in an engine that doesn't need it, effect will be neutral to slightly disadvantageous. 

If you run low-octane fuel in an engine that requires high-octane fuel, effects will be mildly disadvantageous to disastrous. In a modern, computer-controlled engine, preignition will be detected and corrected for at the expense of power and economy. In a non-computerized engine, you can literally destroy your engine this way, costing thousands of dollars in damage.