Q. I like to keep my gas tank full in case of traffic tie-ups, but is it bad for the engine not to let the old gas get used up first?

A. Gasoline does lose octane gradually over time, and old gas sitting in your tank for two months or longer can create residues, but for a car that’s used regularly, that isn’t a problem, says John Ibbotson, chief mechanic at Consumer Reports’ Auto Test Center. “The new gas will mix with what’s already in your tank, and any variance in the octane will be adjusted for automatically by your car’s engine computer.” The octane levels in old gas could be a concern only if you’re storing your car for six months or longer, in which case you can add a gasoline additive (or stabilizer) to preserve it.

And to further help keep your engine clean, consider using a higher grade fuel called Top Tier detergent gasoline, which contains more additives than currently required by the Environmental Protection Agency. Based on recent testing conducted by AAA, Top Tier gas can reduce engine deposits and even clean an engine that has been running on cheap gas. Retailers include corporate giants BP, Chevron, Exxon-Mobil, and Shell, as well as many regional brands.

Check our Guide to Fuel Economy for more. And you can send your questions to ConsumerReports.org/askourexperts.

Editor's Note: This article also appeared in the October 2016 issue of Consumer Reports magazine.

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