ATLANTA (CNN) -- Gas prices declined an average of nearly 15 cents a gallon in the past two weeks but are expected to begin rising as the winter approaches, the publisher of the national Lundberg Survey said Sunday.

The national average for a gallon of self-service regular gasoline was $2.28 on the Oct. 6 survey date, compared to $2.42 two weeks earlier - a decline of about 14 cents, publisher Trilby Lundberg told CNN.

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Gas prices declined a whopping 75 cents during the eight weeks between Aug. 11 and Oct. 6, she added. The average gas price on Aug. 11 was $3.03 a gallon.

The figures are compiled from a survey of more than 7,000 U.S. filling stations.

Lundberg said gasoline supplies typically are plentiful in the summer when demand is highest.

"We are fast using up the overhang [excess] of gasoline supply. Now some refining capability has to be idled for work projects to shift focus from gasoline to home heating oil - so there will be less gasoline produced," Lundberg told CNN.

"There is a turnaround in the making," she said.

As a result of fewer supplies, wholesalers already have begun to pay more for gasoline they distribute to the retailers they serve. Lundberg noted that some refineries are still making repairs from last year's devastating Gulf Coast hurricanes. In addition, imports won't be as plentiful, she said.

For the latest survey period, Des Moines, Iowa, had the lowest average price for a gallon of gasoline, $2.03.

The most expensive gas was in Honolulu at $2.91 a gallon.

Here is a sampling of other cities: Atlanta $2.06; Dallas $2.13; Baltimore $2.17; Milwaukee $2.20; Albuquerque, N.M., $2.28; Long Island, N.Y. $2.36; San Francisco $2.57; Los Angeles $2.58.

Among major oil companies, shares of Exxon Mobil (Charts) and ConocoPhillips (Charts) finished higher Friday, while Chevron (Charts) declined.

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