According to a news release by a California nonprofit organization called Consumer Watchdog, the idea that motorists should fill their tanks in the morning, when gasoline in storage tanks in the ground is coolest, is a myth.

"Many financial advice columns, presenting tips on saving gas, suggest filling up early in the cool of the morning, since gasoline is sold by volume and expands with heat," the organization claims. "It’s absolutely true that U.S. drivers are shortchanged by so-called ‘hot fuel costs’ — to the tune of over $3 billion per year, at today’s prices.

"In southern states, in the summer, gasoline is sold at average temperatures exceeding 80 degrees. Since gasoline expands roughly 1 percent for every 15 degrees Fahrenheit, drivers from Florida to California are getting about 1.5 percent less than they paid for from June through September. That amounts to more than a nickel a gallon.

"It won’t help to buy gas early in the morning, however… That’s because fuel tanks act like thermos bottles, retaining heat."

"Drivers today know that every drop of fuel counts, and they should not have to tolerate getting less than they pay for," said Judy Dugan, research director for Consumer Watchdog.