California ? U.S. retail gas prices continued their downward trend, dropping nearly 15 cents a gallon in the past two weeks, according to a survey released Sunday in Camarillo.

The national average for self-serve regular was about $2.28 on Oct. 6, down from about $2.42 two weeks earlier, according to the Lundberg Survey of 7,000 gas stations across the country.

Mid-grade prices averaged $2.39 a gallon, while premium cost $2.50. The lowest average price in the survey was in Des Moines, Iowa, at $2.03 a gallon for regular.

Gas prices peaked Aug. 11 at $3.02 and have dropped nearly 75 cents since then.