(Reuters) - Gasoline prices in the United States dropped four-tenths of a cent over the past two weeks as crude oil prices fell, ending a long stretch of sustained price increases, according to a widely followed survey.

The national average price of a gallon of regular gasoline was $3.8338 on September 21, down from $3.8376 on September 7, according to Trilby Lundberg, editor of the Lundberg Survey, which covers some 2,500 gas stations nationwide.

While the decline is small enough to be considered "no change," it ends a nine-week run of gas price increases dating to July 13, when the average gallon of gas cost $3.41, Lundberg said on Sunday.

The change was spurred by a fall in crude oil prices caused in part by signals that Saudi Arabia may be willing to supply more oil to the market to lower prices, Lundberg said.

The news could portend a period of price stability assuming relative status quo in crude oil prices.

"Margins on gasoline for refiners and retailers are not only positive but comparatively healthy, so there is no disadvantage at the moment for the industry to withhold a price cut," Lundberg said.

Continued economic struggles in the United States and Europe, as well as an economic slowdown in China, the world's No. 2 oil consumer, also played a role in weakening demand, she said.

The end of the high-consumption summer months brought about lower demand, while the resolution of issues at some U.S. refineries increased supply, Lundberg added.

According to the survey, Chicago has the priciest gas at an average of $4.25 a gallon, while drivers in Jackson, Mississippi, pay the lowest average price of $3.52.

(Reporting By Nick Brown; Editing by Maureen Bavdek)