Real Simpleanswers your questions.

Gasoline can
Credit: James Baigrie

Q. Does it matter what type of gas I put in my car?

Hillary Chazen
Wellesley, Massachusetts

A. It all depends on your engine. Gas stations typically stock three grades of nondiesel fuel, rated according to their level of the flammable chemical octane: Unleaded is the lowest-octane; plus is in the middle; and premium is the highest-octane. High-octane gas won’t necessarily make your car run more smoothly, but it is less likely to ignite prematurely (rather than by your car’s spark plugs, as it should). Spontaneous ignition, while not dangerous, causes an engine to “knock,” or make a rattling noise, and can cause wear and tear. You can try a lower-grade (and cheaper) gas to see if your engine will keep humming. But first check your owner’s manual to determine if the manufacturer requires (rather than just recommends) a higher-octane gas. If so, using a less expensive gas could invalidate the car’s warranty, which means you’ll pay out-of-pocket for costly engine maintenance and repair (a mechanic can tell which grade you’ve been using). And that will cancel out whatever you’ve saved at the pump.