The cost of gasoline in the United States fell 3.6 cents over the last week to $1.059 a gallon, the cheapest level since mid-March 1999, the United States Department of Energy reported on Dec. 17.The pump price of regular unleaded gasoline is down 36 cents from a year ago and is at the lowest level since the week of March 22, 1999, according to the department's weekly survey of more than 800 service stations.The national average price for cleaner-burning reformulated gasoline, sold at about one-third of the stations in cities and smoggier areas, was down 4.1 cents to $1.093 a gallon.A drop in demand for gasoline, due to the sputtering U.S. economy, plenty of supplies and cheaper crude oil, has helped push down fuel prices. Since the Sept. 11 attacks, gasoline prices have fallen by one-third.While prices on the West Coast declined 4.8 cents, the region continued to have the most expensive gasoline in the nation at $1.163 a gallon.Motorists in the lower Atlantic states had the cheapest fuel. The average price in that region fell below $1 a gallon to 99.5 cents.San Francisco maintained the dubious honor of holding the top spot among major cities in fuel costs, with gasoline prices falling 6.7 cents to $1.35 a gallon. Houston had the best prices nationwide, with gas down half a cent to 97 cents a gallon.Gasoline prices in New York City were down 2.5 cents to $1.144; down 5.4 cents in Los Angeles to $1.002; and dropped 9.6 cents in Chicago to $1.106, according to the Energy Department report.The nationwide average price for diesel fuel in the United States fell 3 cents to $1.143 a gallon, down 40 cents from a year ago and the lowest level since early August 1999.Truckers in New England paid the most for diesel fuel at $1.301 a gallon, down 1.3 cents. The Gulf Coast states had the cheapest diesel at $1.096 a gallon, down 3.3 cents.