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Three dollar-a-gallon gas. A fill-up hurts.

Just ask Fabian Torres what it means to his family’s budget.

“I usually get like $4 for school, but now I get like three because my dad has to save money for the gas,” he said.

The higher prices are bad enough but is the Torres family actually getting less gas than they’re paying for?

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“The oil industry -- it's just ripping us off every single day,” Joan Claybrook said.

Claybrook with the consumer group Public Citizen is talking about what happens to gasoline in Houston’s hot weather: It expands, so a gallon of hot gasoline doesn’t provide the same energy as a gallon of cold gasoline.

“There’s less matter, less actual chemicals, molecules in a hot liquid per volume than there is in a cold liquid,” chemist Andrew Barron said.

Barron is not only is a chemist at Rice – he races cars.

“You actually want it colder to create more horsepower,” he said.

Hot weather and hot gasoline mean less horsepower per gallon.

So what’s all this mean when you fill up?

By one estimate, it amounts to one-third of a gallon of gas per 20 gallon fill-up. One-third of a gallon of gas you’re paying for but not getting. At today’s prices, that would be about $1 a fill-up.

Multiply that by the millions of gallons sold by the big oil companies and consumer advocates figure it means $2 billion a year in undeserved profits.

“Oh, it's serious money,” Claybrook said. “That’s why they’re fighting like mad not to have any change in the measurement device.”

Proposals to change how gas is measured at the pump so you’d get more during hot weather were shot down just days ago in the Texas Legislature.

Hawaii passed such a measure years ago.

Oil companies say lots of things effect gasoline but as a practical matter, a gallon is a gallon.

And station owners say requiring them to adjust pumps depending how hot it is is ridiculous.

“But to regulate and maintain that thing will end up costing us more money,” said Karim Dhukani with the Greater Houston Retailers Cooperative Association.

“That variation is so small, I don’t think it will make any difference to the consumer,” Barron said.

Maybe not, but at these prices you’d want every drop you pay for.