UPDATED -- Diesel fuel and gasoline prices in the U.S. continue moving lower with the national averages posting their largest weekly drops in quite a while.

New U.S. Energy Department figures show a decline of 3.5 cents with the average cost of on-highway diesel falling to $3.698, marking the 15th consecutive week the price has fallen or been unchanged from the week before and hitting it its lowest level since July 2012. It has also fallen for five consecutive weeks.

Compared to a year ago the price is 18.8 cents lower and a high so far this year of $4.021 in March.

Prices declined over the past week in all sections of the country ranging from a little as 2.5 cents in the Gulf Coast region, for an average of $3.637 and the second least expensive average price in the country, while the biggest drop was 4.6 cents in the Central Atlantic states for an average of $3.784.

The highest average price in the country is in the West Coast region at $3.891, down 4.2 cents from last week, while the least expensive is in the Midwest at $3.636, a decline of 3.4 cents during the same time.

Compared to a year ago, regional prices are down between almost 14 cents to as much as nearly 22 cents.

The average regular grade gasoline price posted an even bigger decline that diesel, falling 9.2 cents over the past week to $3.207 per gallon. Compared to the same time a year ago it is 14.7 cents less.

Gasoline prices declined in all sections of the country over the past week and range from a low of $3.008 in the Gulf Coast region, down 10.1 cents, to a high of $3.536 in the West Coast region, a decline of 7.4 cents.

Oil Prices Sink

The reason for the drops is due to a big decline in oil prices recently, including on Tuesday with benchmark crude falling $3.90 in New York to $81.84 per barrel by the end of the trading day. This is its biggest one-day percentage decline in about two years and the lowest closing price since June 2012.

Overseas, Brent crude also fell to close at $86.24 on the day, its lowest settlement price since December 2010.

Both were pushed lower by a report from the International Energy Agency forecasting world oil demand to weaken into 2015 while oil production from U.S. shale basins will increase this November by more than 100,000 barrels per day from this month.

DEF Prices

On a related note to trucking expenses, a recent survey shows the average cost for diesel exhaust fluid at the pumps is stable, but it's down when purchased in prepackaged containers.

The national average for DEF pump prices in the U.S. remained unchanged at $2.79 per gallon for the sixth consecutive month in September according to Integer Research, while the Canadian average stayed at CA$0.80 per liter for the fourth consecutive month.

Average 2.5 gallon jug prices decreased by 11 cents per gallon to $5.79 per gallon in the U.S. and remained stable at CA$1.84 per liter in Canada.

Update adds gasoline prices.