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Octane Rating: Grace Under Pressure

What’s the difference between regular and premium gas? Check out Groupon’s guide to the cryptic numbers on the gas pump.

When deciding between regular and premium gasoline, the only thing that matters is the number on the pump. Not the price, though—the octane rating. For each grade of gasoline, a yellow label sports a bold number—usually 87, 89, or 93—that corresponds to the fuel’s concentration of octane. The term octane stems from the refining process. On a molecular level, crude oil can be broken down into chains of different lengths, such as methane, which has one carbon atom, or propane, which has three. Blending different levels of these fuels changes the way the gas reacts to compression. During the engine’s compression stroke, a cylinder of air and gas compresses into a much smaller volume, after which the spark plug ignites the fuel. If the gas is compressed too much, however, it can sometimes trigger an ignition before the spark plug intends. Since octane is highly resistant to compression, the octane rating represents how much compression the fuel can handle before it ignites on its own. An 87 rating, for example, means that the gas contains 87% octane and 13% heptane—a more volatile chain.

Not every engine can handle the same octane rating, and choosing the wrong fuel can sometimes do unnecessary damage. High-performance engines, in particular, have higher compression ratios in order to generate more horsepower. High-performance engines are typically found in luxury vehicles as opposed to everyday cars, however, so unless your owner’s manual demands premium gasoline, don’t bother filling up on more expensive fuels. The regular octane rating is more than enough to keep engines from dying of thirst.