air/fuel mixture. Aka “knocking”. If you have an engine that is designed to run at a higher compression level (usually a sports car or luxury car, in my case it’s a 25-year-old Lexus SUV) it requires higher-octane gas because 91 won’t ignite at a compression level that will ignite 86 or 89. So you avoid knocking that you might get with a lower octane.

It’s important. Knocking makes your car inefficient, which may sound like a fair trade off for cheaper gas, but it will also physically damage your combustion chamber and cylinder heads. Eventually it can lead to catastrophic engine failure.

My old lady is almost to 300K miles. These particular motors are incredible, but I don’t think she would have gotten here if I ran lower octane the whole time.