What is octane rating? What is cetane rating? When purchasing gasoline or diesel fuel you’ve likely noticed the selection of numbers available to purchase, whether it’s 87, 91, 98, etc. The numbers shown are the octane rating for gasoline, or the cetane rating for diesel fuel, and these numbers indicated properties about the fuel you’re buying. The octane rating tells you how resistant a fuel is to knock; another way of thinking about this is how much can you compress that fuel, mixed with air,