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 chair, 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 oppose forcing stations to adjust their pumps as costly, and 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 over-collection 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 as a regular gallon 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 five degrees warmer than the federal standard, according to a study analyzed by Dick Suiter of the National Institute of Standards and Technology. But it’s worst in southern and western states where the temperatures are the most consistently warm.

According to the National Oceanic and Atmospheric Administration the average U.S. temperature in May was 63 degrees; average for all of 2006 was 55 degrees.

The impact isn’t lost upon Carl Rittenhouse, a carpet worker from the north Georgia town of Chatsworth.

“You can tell the difference between the time you fill up in the morning or night, or if you fill up in the middle of the day,” said Rittenhouse, who joined one of the lawsuits. “All you have to do is look at the fumes.”

The debate is now reaching Washington.

Sen. Barbara Boxer, D-Calif., recently urged California lawmakers to take action. And Rep. Kucinich earlier this month called a hearing on the issue, calling it “Big Oil’s double standard.”

“People are paying for gasoline they’re not getting,” said Kucinich, who is running for president.

Lawmakers don’t have to look very far for possible solutions.

In frigid Canada, where cold temperatures were giving consumers an edge, many gas stations voluntarily backed a program to add pumps that automatically adjust volumes based on temperature.

During the energy crisis in the 1970s, tropical Hawaii decided to set a base fuel temperature of 80 degrees, meaning that consumers there get more bang for their buck because retailers now dispense 234 cubic inches of gas per gallon rather than 231.

The federal government is considering a similar change as well. The National Conference on Weights and Measures is to vote in July on whether to allow temperature regulation by retailers.

The coming decision is worrying some fuel distributors, who say the new equipment could force some independent dealers out of business. NATSO, a trade group representing truck stop owners, estimates that each retrofitted pump could cost between $1,500 and $3,800.

“The average truck stop has 20 pumps,” said Mindy Long, a spokeswoman for the group. “The burden on them would be phenomenal.”

NATSO and other gas retailers have formed a group called PUMP – the Partnership for Uniform Marketing Practices – which is calling for more studies on the issue before taking any action.

They have a powerful ally in Rep. Bart Gordon, the Tennessee Democrat who chairs the House Committee on Science and Technology. In a May letter to the National Academy of Sciences, he suggested the idea of retrofitting pumps may be “premature.”

The trucking companies and motorists behind the lawsuits hope they could force politicians to act quicker.

“You’re not getting as much as what you’re paying for, really,” said Rittenhouse, the north Georgia motorist. “Most folks don’t have a clue. But it’s costing them.”