The nationwide price for diesel fuel fell 3 cents to $1.143 a gallon in the past week, down 40 cents from a year ago and the lowest level since early August 1999, the Energy Department reported. 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. The cost of U.S. gasoline fell 3.6 cents over the last week to $1.059 a gallon, the cheapest level since mid-March 1999.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 petroleum demand due to the slowing U.S. economy, plenty of gasoline supplies and cheaper crude oil has helped push down fuel prices. Since the Sept. 11 attacks, gasoline prices have fallen by one-third.