Gas pumps are commonly labeled regular, mid-grade, or premium, which for the most part relates to the octane rating. Octane is defined as a resistance to detonate. The higher the octane number, the more the fuel resists detonation under pressure. The more the fuel resists detonation, the more complete the combustion is when the fuel ignites. Usually higher compression engines need higher octane fuels. Common octane numbers are regular 87, mid-grade (or plus) 89, and premium 93. The octane requirement depends on the engine design, the altitude the vehicle is driven, and driving habits. Most automobiles require a minimum of 87 octane, but always check the owner’s manual.