ATLANTA (CNN) -- Gas prices tumbled 24 cents during the past two weeks to an average of $2.42 a gallon of self-serve regular, a national survey said Sunday, posting the third straight two-week decline.

The decline brings the drop to 66 cents over the past six weeks, said Trilby Lundberg, whose "Lundberg Survey" tallies prices every two to three weeks at about 5,000 gas stations around the country.

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But the trend may not continue, she said.

"I wouldn't be surprised if price cuts come to a screeching halt very soon," she said. The supply cushion, which had been inflated at the end of the summer, when refiners and importers maxed out refinery runs and imports, will grow thinner, she predicted.

"As the supply and demand situation stands, I do think that any subsequent price cuts at the pump will be much smaller," she said.

In fact, cuts may be replaced by increases, she said, noting that prices in some western and Midwestern markets for unbranded gasoline have turned upwards.

Drivers in Des Moines paid the least last week at $2.07 a gallon; Honolulu drivers paid the most, at $3.03.

Here are some other prices:

Los Angeles: $2.76

Salt Lake City: $2.76

Seattle: $2.67

Washington: $2.52

Manchester, N.H.: $2.42

Atlanta: $2.21

Houston: $2.20

Saint Louis: $2.20

Indianapolis: $2.08

The report comes as oil prices sank below $60 a barrel Monday for the first time six months.

Major oil companies like Exxon Mobil (Charts) and Conoco Phillips (Charts) as well as big refiners like Valero Energy (Charts) could see profits under pressure if oil and gas prices keep falling.

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