The average U.S. price of a gallon of gasoline dropped six cents over the past two weeks, according to the Lundberg Survey of fuel prices on Monday. According to the survey, the price of a gallon of regular gasoline is at $3.78. Midgrade gasoline costa an average of $3.94 a gallon and premium is at $4.06. Diesel dropped six cents to $4.09 a gallon. Of the cities surveyed, Memphis, Tennessee has the nation’s lowest average price for gasoline at $3.34, while San Francisco, California has the highest price at $4.36. In California, the lowest average price was $4.24 in Fresno, while the average statewide price for a gallon of regular was $4.33, up about 16 cents.