If you've been spending extra to fill your vehicle with the best gas money can buy, you may want to reconsider. In reality, buying the grade, or “octane rating,” listed in the owner’s manual or inside the fuel-filler flap will get the job done.

The octane rating measures how well a fuel resists knock, or pinging—essentially uncontrolled combustion—in an engine. The most common levels are 87 (regular), 89 (midgrade), and 91-93 (premium). The higher the number, the greater the resistance to knock.

You gain nothing except a thinner wallet by buying a higher octane than needed. Engines today have a sensor that retards engine timing (basically, pulls power out) if it hears knocking. So if the engine needs 93 and you use 87, the sensor will cut power to prevent engine-killing knock.

If you don’t need all the performance your engine’s capable of, you can probably get away with cheaper fuel. But—and this is a big but—you’ll likely get worse fuel mileage, too. So the lower cost could be outweighed by more frequent fill-ups.

For high-performance engines built before knock-sensor tech, use the highest octane. And don’t be fooled by ads about additives, the EPA already requires them in all gas to clean vital parts like fuel injectors and intake valves.

John Dinkel is an auto engineer, lifelong racer, and a columnist for Men's Fitness. Follow him on Twitter.