ATLANTA — It's not just increased demand that sends summertime gasoline prices soaring. It's also the increased temperature.

As the temperature rises, liquid gasoline expands and the amount of energy in each gallon drops. Since gas is priced at a 60-degree standard and gas pumps don't adjust for any temperature changes, motorists often get less bang for their buck in warmer weather.

Consumer watchdog groups warn that the temperature hike could end up costing consumers between 3 and 9 cents a gallon at the pump.

The effect could cost U.S. drivers more than $1.5 billion in the summertime, including $228 million to drivers in California alone, according to the House Subcommittee on Domestic Policy, which recently addressed it in hearings. The committee's chairman, Rep. Dennis Kucinich, D-Ohio, has long been an advocate on the issue and has new clout as a member of the congressional majority.

Gas retailers say forcing stations to adjust their pumps would be too costly, and they asked Kucinich to call off the hearings and wait for more studies.

The issue has driven trial lawyers to fire off as many as 20 federal lawsuits accusing retailers of using simple physics to take advantage of consumers. Challenges have been filed in Alabama, Arkansas, California, Florida, Kansas, Missouri and New Jersey, among other states, and some are seeking class-action status.

The latest lawsuit, filed last week in federal district court in Georgia, claims that distributors have been "unjustly enriched" by tens of millions of dollars. They did so by paying taxes on the fuel based on the colder industry standard but pocketing the taxes collected from customers when the temperature soars, it alleged.

"I don't believe gas retailers should collect more in purported taxes than they pay the government," said Bryan Vroon, one of the attorneys in the Georgia suit. "Gas prices are high enough without the overcollection of taxes."

The "hot fuel" effect is a matter of simple physics.

Almost a century ago, the industry and regulators agreed to define a gallon of gasoline as 231 cubic inches at 60 degrees. But as the mercury rises and gasoline expands, it takes more than a gallon of gas to produce the same amount of energy. The opposite is true when gasoline contracts in colder weather.

U.S. gas retailers ignore the temperature swings and always dispense fuel as if it's 60 degrees. As a result, gas is an average of about 5 degrees warmer than the federal standard, according to a study analyzed by Dick Suiter of the National Institute of Standards and Technology.