Despite spot price increases in areas of the East Coast, the average nationwide price for diesel fuel continues to be on a downward slide.

According to the U.S. Energy Information Administration, the national average price of diesel has dropped to roughly $4 a gallon. In mid-October it was $4.15 a gallon.

A year ago, the U.S. average price of diesel fuel was $3.88 a gallon.

In the aftermath of Superstorm Sandy, however, prices increased marginally for diesel in the New England and Central Atlantic areas.

Across the nation, the largest drop in the price of diesel has been in California. The average price per gallon has dipped to around $4.20 in the Golden State, but the California price remains the highest in the U.S.

The price of gasoline has joined diesel on the down slope.

Average gasoline prices nationally have fallen to under $3.50 per gallon. The cheapest gasoline in the nation can be found in the Gulf Coast, according to the energy agency, where the average price per gallon is around $3.25. In some places, its around $3.

Meanwhile, the U.S. Department of Energy has lowered its projections for crude oil prices for the fourth quarter of this year and expects diesel fuel to average $4 per gallon for the quarter.

The agency also is predicting crude oil to remain about the same in 2013, and diesel prices to be slightly lower. The agency expects on-highway diesel fuel retail prices to average $3.83 per gallon in 2013.

The agency forecasts gasoline to average $3.44 per gallon in 2013.

DEF prices elevated

Brighton, Mich. -- The average price of U.S. diesel exhaust fluid rose 4 cents during October, to an average of $2.80 per gallon, reports DEF Tracker.

The October price is the second-highest average pump price for diesel exhaust fluid during the past 12 months, behind June at $2.88 per gallon.

The report also notes that bulk DEF dispensing facilities at truck stops reached its highest level since DEF Tracker was launched in October 2011, with 77 new locations coming on line during October 2012.

DEF Tracker is a monthly subscription service giving diesel exhaust fluid price references for full truckload and less than truckload bulk deliveries, tote refills and packaged products, and pump prices in every U.S. state and Canadian province.