system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the differences between Regular and Premium gas and how does it affect different vehicles?
gpt
Octane rating, which is the fuel's resistance to ignition. So "regular" gas will ignite at a lower temperature.

Compression of air and fuel is an important part of the internal combustion engine's cycle. Compressing air quickly makes a lot of heat as all those molecules get smashed into each other. In some cars, like modern or high-performance ones, this heat is enough to ignite gas with a low octane rating. This is very bad for the engine. Conversely, putting premium in a car with lower compression will just eat your money and actually cause a loss in efficiency, as the fuel doesn't get hot enough to burn fully.