CAMARILLO, Calif. — The average price of a gallon of regular-grade gasoline fell a nickel nationally over the past two weeks to $2.51. Industry analyst Trilby Lundberg of the Lundberg Survey said Sunday that the drop comes as distribution systems continue to get back to normal following disruptions from late-summer hurricanes along the Gulf Coast. Gas in San Francisco was the highest in the contiguous United States at an average of $3.07 a gallon.