Gas prices may be dropping to $3 a gallon again, some experts believe, after prices fell 6.4 cents in the last two weeks.

The average price of a gallon of regular gas in the U.S. is down 31 cents from this time a year ago, the latest Lundberg survey from Sept. 20 shows. The industry poll included about 2,500 retail stations in the 48 contiguous states.

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According to the survey, the cheapest gas was $3.14 in Charleston, S.C., while the most expensive pumps were in San Francisco, where drivers paid $4.01 per gallon.

Trilby Lundberg, the survey's editor, said the drop in gas prices is a result of the declining price of crude, which fluctuated because of the recent diplomatic progress in Syria and Iran, according to NBCNews.com.

GasBuddy.com, an online tool that monitors gas prices throughout the U.S., agreed.

"Supplies are adequate, and indeed gasoline now typically sells for just a few dollars more than raw crude costs. [Prices will] go below $3.00 anytime soon," GasBuddy.com's chief oil analyst Tom Kloza told NBCNews.com.

Other experts, however, say the dip is just the typical September decent.

"It’s normal for prices to fall in September," Mark Jenkins, American Automobile Association spokesman, told NBCNews.com. "The summer driving season is over, demand is down, and refineries are switching to the cheaper winter-blend gasoline. Prices could continue to fall another 5 to 10 cents before the end of the month, but it’s unlikely."

The price of a gallon of regular gas has not dropped below $3 in three years, according to AAA.

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