The average price of a gallon of diesel fuel was $3, according to weekly figures released Monday by the U.S. Department of Energy.

That's the highest average reported since May, when they reached as high as $3.127, according to the U.S. Energy Information Administration figures for retail on-highway diesel prices. It's 41.8 cents higher than a year ago.

The highest average prices were seen in the West Coast region, at $3.165. The lowest were in the Gulf Coast region at $2.92.

Meanwhile, light, sweet crude oil for November delivery ended the day at $81.47 a barrel on the New York Mercantile Exchange Monday. Oil futures hadn't ended above $80 a barrel since early August until late last week. A drop in U.S. fuel inventories and a weakening dollar helped send oil prices higher last week. But the dollar has bounced back a bit compared to other world currencies.