When we stop through the gas station, we pretty much always see that there are three types of gas: regular, midgrade, and premium. However, a lot of us still have the concept of “unleaded” gas in our vocabulary as a choice when in reality all gas is unleaded. So, what’s the difference between these types of gas and why do we still use the term “unleaded” – and where did it come from?

First, the difference between the different types of gas is the octane level of each type. At the risk of getting too science-y, I will speak as plainly as possible. Basically, everything is made of molecules, right? In gasoline, there are certain molecules called hydrocarbons, which are from the oil that is used to produce the gas. One type of hydrocarbon that’s in gas is isooctane (aka just plain old “octane”). Regular old hydrocarbons are pretty good at just exploding whenever they feel like it, presumably, but isooctane is resistant to that spontaneous combustion. The more isooctane in the gas, the higher the octane rating. The higher the octane rating, the less chance your engine will “knock,” which is a result of the gas exploding too soon. See where I am going with this?

Your next question might be, “does my car need this higher octane gas?” The answer to that is, well, it depends. Do you have a super aggressive engine under the hood? Does your owner’s manual say to fill up with premium gas? If not, you’re fine with the regular stuff. If you put higher octane fuel into your vehicle when it doesn’t need it, you’re basically just wasting money.

And finally, a quick history on “leaded” versus “unleaded” gasoline. Before they got into the whole natural octane thing, car makers didn’t know how to minimize the knock in their vehicles’ engines. That is, they didn’t know until someone remembered that tetraethyl lead could boost the octane ratings while keeping the cost of gas down – AND it got rid of the “knock.” Sure, it was poisonous and sure, it led to a lot of deaths due to lead poisoning between the 1920s and the 1990s, when leaded gas was banned – but hey! Cheap gas!

Nowadays, lead is banned from being in gas and pumps still have to say specifically that the gas is unleaded, and now you know why!