Even if you refuel your car on a regular basis, the signage at gas stations can be hard to decode. In addition to confusing gas prices, motorists encounter a set of numbers each time they visit the pump. Whether you automatically press the button with the 87 on it or opt for a higher fuel grade, you should know what exactly those numbers measure.
According to the U.S. Energy Information Administration, the numbers on the yellow buttons for regular, mid-grade, and premium gas indicate the fuel's octane rating. The regular gasoline most people purchase typically has an octane rating of 87. The rating for mid-grade gas usually ranges from 89 to 90, and for premium, it falls between 91 and 94.
Simply put, octane ratings measure the stability of fuel. To make a car run, an engine's spark plug lights a flame that progressively burns through all the fuel in the cylinder. This process is called controlled combustion. Under certain conditions, the pressure of this combustion combines with rising temperatures to ignite unburned fuel without the flame from the spark plug. This uncontrolled combustion, also called pre-ignition or knock, increases pressure in the cylinder, potentially causing damage to the car's engine.
Knock (named so because of the literal knocking sound the overworked engine makes) is less of a problem now than it used to be. Today, an oxygenate is added to commercial fuel to prevent it from igniting prematurely. Octane ratings indicate how high the fuel's oxygen content is, with the lowest numbers being most prone to uncontrolled combustion. To measure a fuel's research octane number (RON), the gasoline is tested in an idling vehicle with low air temperature and slow engine speed. It's tested under higher temperatures and high engine speeds when measuring the motor octane number (MON). These two numbers are averaged together to determine the octane rating.
Oxygenated fuel, along with computerized ignition and knock-detecting sensors, has greatly reduced uncontrolled combustion in modern cars. Though 87-octane gas is perfectly suitable for most people, certain vehicle owners should opt for premium. High-grade fuel is the most efficient choice for high-performances cars that use higher compression ratios, turbocharging, and down-speeding. And if your car is especially luxurious, it may be worth paying the extra money for gas that's least likely to cause engine damage.