Diesel prices dropped for the third week in a row and are the lowest they've been since late February. Weekly figures from the Department of Energy show the average retail price of on-highway diesel is $3.937 a gallon, down 6.2 cents from last week and down more than 11 cents over the past three weeks.

But it's still 85.6 cents higher than it was a year ago.

Prices still hovered at the $4 a gallon mark in the New England region, where the average price was the highest. The lowest average prices were reported in the Gulf Coast region at $3.806.

Crude oil futures prices, meanwhile, settled higher Monday, closing at $87.88 a barrel for light, sweet crude on the New York Mercantile Exchange. It was the highest price since Aug. 3.

International forecasts have lowered their outlook for U.S. demand for oil,

According to the Wall Street Journal, analysts said that amid the uncertain global economy, underscored by the plunge of U.S. crude oil prices to below $75 a barrel after Standard and Poor's downgraded the U.S. credit rating, OPEC is likely to be extremely vigilant about defending prices in order to meet member nations' budget needs.