Though the price of gas has come down recently, the cost of gasoline has been pumped up for some time. Now comes new evidence that consumers may not be getting their money's worth when they fill up.

When stored gas heats up, it expands -- diluting its energy, and reducing miles per gallon.

The amount of energy per gallon you lose as gets hotter is an exact scientific measurement. More simply, it's estimated that on a hot day you could lose the equivalent of about a quart of gas per fill-up.

Nationwide, critics estimate this has given up to $2.3 billion of extra profit to oil companies. Why? Because unlike for the average consumer, when most oil companies, distributors and stations purchase gas, they are compensated for gas that is above 60 degrees. Special equipment adjusts the amount pumped according to its temperature.

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Figures compiled by the government found gas stored in tanks at Florida stations averaged over 82 degrees -- the highest in the continental U.S. The government researchers say temperature compensation is the fairest way to sell gas.

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But the oil industry says the high cost retro-fitting existing gas pumps would be passed on to customers -- a cost they say would far exceed what customers would save by adjusting gas temperature.

Only one state -- Hawaii -- sells temperature-compensated gas. One tip: buy gas in the morning, when it's cooler -- you'll actually get more energy per gallon if the gas is below 60 degrees.

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