What do the Numbers on a Gas Pump Mean?

You've probably stood at the gas pump thousands of times in your life without considering what the numbers on each of the pumps represent.

In a nut shell: the numbers on the pump are an octane rating that measures the different volatility levels of gasoline. The numbers measure volatility in reverse of what you would expect. For example: A premium fuel with an octane rating of 93 is less volatile fuel, than a lesser grade fuel with an octane rating of 87.

If you're anything like me, that doesn't mean much without further explanation. So let's delve in and figure out just what volatility means. The word "volatile" is misunderstood by a lot of people. Our Service Manager and Certified Master Technician, Peter Badia, has another way of thinking about it: Octane refers to how fast the gas burns.

Think of volatility as a measure of explosive power. Something highly volatile will "explode" quicker, and with more force than something less volatile. Different engines require different levels of volatility in gasoline to operate properly.

Most engines either use premium grade fuel, or lower grade fuel. If you use premium fuel in your car, you'll notice it's usually the largest number on the gas pump. It seems counter-intuitive, but remember the larger the number, the less volatile the fuel is.

In a premium engine, you don't need highly volatile gasoline, because you're talking about a high performance engine - the pistons compression ratio is much greater than that of a standard engine, and therefore doesn't need highly volatile fuel.

If you have an engine that requires lower grade fuel, the octane rating of the gasoline you use is smaller, but it's more volatile. This is necessary because an average performance engine has pistons with lower compression ratios - more volatility is needed in the controlled explosion to power the engine.

Pre-ignition is the result of putting lower grade fuel in a high performance engine. Lower grade fuel won't allow a high performance engine's piston to fully compress because it's too volatile. The car might run on regular fuel, but it's not great for the engine, you lose fuel economy, and it will run noticeably worse.

The reverse is true if you put premium fuel in a standard performance engine. The fuel won't have enough volatility to get the engine running optimally. In other words, the fuel doesn't have enough juice to get the engine performing at the right level.