Amidst some of the lowest crude oil prices since 2009, the average price of a gallon of on-highway diesel fuel continued its more than 2-month streak of price drops, according to the latest numbers from the U.S. Energy Department.

The average price dropped for the 13th straight week, sliding 5.4 cents nationally to an average price of $2.561 per gallon. Prices are significantly lower than this period last year, down $1.26 per gallon.  

Regional prices were down across the board, with the largest drops coming to the West Coast and New England, where prices fell by 6.4 cents. The smallest decrease occurred in the Rocky Mountain region, falling 3.5 cents.

The price of a gallon of regular gasoline was down 7.9 cents for the week, largely erasing the 8.7-cent increase the week prior. Last week’s increase was mostly due to the partial shutdown of a BP refinery in Indiana, which spiked gas prices in the Midwest.

This week’s largest drop by region occurred on the West Coast with an 8-cent price decrease, and the smallest decrease was found in the Rocky Mountain region at 1.5 cents.

Prices could see even further drops, as crude oil prices dove to six-year lows. West Texas Intermediate crude futures ended the day under $39 a barrel Monday on the New York Mercantile Exchange, at $38.24. Similarly, Brent Crude oil traded at six-year lows coming in at $42.69 per barrel, according to a MarketWatch report.

Oil supply continues to outstrip demand globally, causing prices to precipitously decline throughout much of 2015. Total global liquids inventories have grown by 2.3 million barrels per day through the first seven months of 2015. By comparison, the price of barrel of crude oil was trading at or near $100 a year ago this month.