When you fill up your car with gas, you probably have to make a choice: do you pick the cheaper gas with the lower number?

Or do you go for the more expensive stuff with the higher numbers? Since a lot of gas stations call them “regular” and “premium”, you might think that it would be better for your car to shell out for the gas with the highest number.

But that’s not necessarily true. Using the wrong type of gas — with a number that’s too low or too high — can be worse for your engine, the environment, and your wallet. That number on the pump is called an octane rating, and it tells you how much the gasoline can be compressed before it’ll just ignite.

Which is important, because if gasoline ignites too soon it can really mess up your car’s engine. In the typical engine you’ll find in a car that runs on gasoline, a piston slides out of a chamber to let fuel in, . Then, a spark gets the gasoline burning, and the hot gas pushes the piston back to let the flame’s byproducts out and let more fuel in.

That all has to happen in exactly the right order. If the gasoline ignites before the spark, the explosion’s shock wave tries to push the piston out while it’s still moving in to compress the fuel. When the energy can’t be used to push out the piston, it rattles the sides of the chamber to create a knocking or pinging sound, in what’s fittingly known as “engine knock”.

The octane rating tells you how much the fuel can be compressed before it’ll ignite without a spark. Basically, the higher the number, the more stable the fuel. Octane ratings are calculated different ways in different countries, but they always compare the gas’s stability to a mixture of two molecules: heptane and octane.

They’re both just chains of carbons attached to hydrogens — the only difference is that heptane, with 7 carbons, is really unstable, whereas octane, with 8 carbons, is really stable. So comparing gasoline to a mixture of those two molecules is a good way to measure its stability. But just because premium gasoline is more stable doesn’t mean you should suddenly switch to using it!

Today’s engines are designed with specific ratings in mind. Engineers know that high-octane fuel can take more compression without igniting, so high-octane engines compress more than low-octane engines, and low-octane fuel can cause them to knock. But low-octane engines don’t compress enough for high-octane fuels, and there isn’t enough time between the spark and the gas being swept out of the chamber for all the fuel to be used up.

That means some high-octane gas gets wasted if it’s in the wrong engine, and once it’s out of the engine unburned, it can mess up things like the system that should be cleaning your exhaust. So it’s just better to use the kind of gas your car was designed for. Which you can probably find written somewhere in that owner’s manual you never read.