system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what the difference between octane 98/95/91?
how does it effect the car/mileage/engine?
gpt
In a gasoline engine, the gasoline is meant to be set on fire by spark plugs at a specific time. But because the inside of the engine is very hot, and the gasoline is under pressure, sometimes the gasoline will catch fire before it is supposed to. This is called knocking, and it is bad for the engine. It is also bad for performance and mileage.

The octane rating of a blend of gasoline tells you how much it resists knocking. The octane rating you need depends on your engine, and as long as you are using gasoline with the right octane rating, you won't get better performance from a higher rating. If you have a car that was not made to go very fast, then you probably can use the cheapest gas. If in doubt, read your car's manual.