Updated

U.S. average retail gasoline prices fell slightly over the past two weeks and could ease a bit more in coming weeks amid larger U.S. stocks of gasoline and flat demand, according to the latest nationwide Lundberg survey.

The national average price for self-serve regular unleaded gas was $2.9328 a gallon on May 19, a decline of about 1.5 cents per gallon in the past two weeks and the first drop since February, according to the survey of about 7,000 gas stations.

At $3.40 a gallon, San Diego had the highest average price for self-serve regular unleaded gas, while the lowest price was $2.59 a gallon in Wichita, Kansas, the survey said.

"Demand has been flat because of high prices, while gasoline supplies have grown," survey editor Trilby Lundberg said in an interview. Other trends that have caused a slight downturn in prices were higher imports of gasoline, lower prices of crude oil and higher utilization of refining capacity, she said.

Gasoline prices could drop "a few more cents" in coming weeks unless crude oil prices surge again or refining problems develop, she said.