What’s the problem?

Gasoline and diesel expand or contract, depending on the fuel’s temperature. A gallon sold at the government-standard 60 degrees has more energy in it than a gallon sold at 80 degrees. U.S. pumps can’t tell the difference, so consumers pay the same price for both.

Why should I care?

Gasoline expands or contracts 1% for every 15-degree change in the fuel’s temperature. Diesel volumes change 0.6% per 15-degree change. The difference seems small, but it adds up fast in California, where fuel temperatures can be much higher than 60 degrees and prices are steep. It’s a bigger hit for truckers, whose rigs gulp 20,000 gallons or more of diesel a year.

How big is the cost to consumers?

The overpayment depends on the temperature of the fuel and the retail price of gas, and both are in constant flux. A consumer group believes the problem costs California drivers an extra 3 cents a gallon.

Should I buy fuel in the morning or when temperatures are cooler?

No. The delivery temperature is key, because most fuel sits in underground tanks that act like big Thermos bottles. Even if a station receives a load of gas at 5 a.m., if it’s coming straight from the refinery, the fuel will be hot and stay that way no matter how cool the morning.

Is there a fix?

Yes. Gas stations could install equipment that recalculates volumes based on the fuel’s temperature. In California, similar equipment is in use for wholesale fuel purchases.

Why haven’t regulators stepped in?

Federal and state officials believed the fuel at gas stations stayed close to 60 degrees because it was stored underground and that any variations would even out over the year or be too small to matter to consumers.

What’s changed?

Gas stations switched to the double-walled underground tanks. Also, an industrywide shift to fewer, higher-volume gas stations means that fuel spends less time underground. And rising prices have added urgency to getting a fair deal at the pump.

What’s being done?

California officials are recording fuel temperatures to quantify the problem. And government regulators are reviewing a proposal that clears the way for fuel retailers to install temperature-adjusting devices.

— Elizabeth Douglass