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SCOTT JAGOW: Miss Caudle was my seventh grade science teacher. She told me that when liquids heat up, they expand. And I think I said, when am I ever gonna use that on the public radio show I’ll be hosting when I grow up? It turns out today is that day. The liquid is gasoline. A group of drivers claims we’re getting ripped off at the gas pump when the weather’s warm. More now from Dan Grech.

DAN GRECH: Gasoline is supposed to be sold at 60 degrees Fahrenheit. That’s according to a century-old agreement between gas companies and regulators.

A class-action lawsuit filed this week by drivers in seven states accuses gas companies of breaking that promise.

Joan Claybrook is president of the consumer advocacy organization Public Citizen.

JOAN CLAYBROOK: The fuel, when it’s warmer, it expands. So you’re paying for a little bit more fuel than you’re getting into your tank in terms of energy.

Hot fuel costs individual drivers a few cents extra per gallon.

CLAYBROOK: Pennies per person, but then if you drive a lot it really does add up.

She says all told hot gas costs U.S. drivers $2 billion a year. Gas stations can install machines to adjust for warm gas.

Of course, that’s an expense that would likely be passed onto consumers.

I’m Dan Grech for Marketplace.