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. But drivers fare worst in Southern and Western states where the temperatures are the most consistently warm.

Increased demand also sends gas prices higher during the peak summer travel season, so the effect of paying more for less in the warmer months is more pronounced.

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 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 upcoming 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 to $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."