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Will premium gasoline make your car run better? Can regular gasoline damage your engine? Does midgrade give you better gas mileage? Motorists often ask these questions — and come up with the wrong answers. This short article will help you decide what kind of fuel to put in your car.

The Short Answer

Use the cheapest fuel for which your car’s engine is designed. You’ll find the minimum octane rating in the owner’s manual, and cars that require premium fuel will usually say so on or near the gas cap. If your car specifies premium fuel, use the good stuff. Otherwise, there’s (usually) no reason to buy anything but regular 87-octane fuel. (See the section on buying fuel at high altitude for the exception.)

The Long Answer

Gasoline is rated by octane. Generally, regular fuel is 87 octane, premium is 91 or 93, and midgrade is (surprise, surprise) somewhere in the middle.

To understand what octane means, we first have to look at the concept of preignition. Engines compress a mixture of air and fuel and ignite them with a spark. Under certain conditions, the fuel-air mixture can ignite early, which makes a knocking sound not unlike a coffee percolator — this is called preignition. Nowadays, most cars have knock sensors that prevent preignition, so you’ll rarely hear it.

Octane measures the gasoline’s resistance to preignition; the higher the octane, the less likely it is to detonate. Contrary to what some believe, high-octane gasoline has the same energy content as low-octane gasoline.

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