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,

People think higher octane equals better quality fuel. Wrong. Octane rating prevents pre-detonation, not performance gains. Lower octane than your engine requires causes knock. The fuel air mixture combusts prematurely before the engine wants it, causing long term wear and damage. But using higher octane than required provides zero benefit unless your engine has adaptive timing that adjusts based on fuel quality. Most cars don’t. Using 91 in an 87 octane car wastes money. Using 87 in a 91 octane