For the seventh straight week, the average price of diesel fuel decreased while gasoline saw a slight increase, according the most recent numbers from the Energy Department.

The average price of a gallon of on-highway diesel fuel dropped 4.9 cents last week, settling at $2.235 and once again setting a new low for 2015. At the end of a year in which crude oil prices consistently fell, the price of diesel today is 97.8 cents cheaper than it was a year ago.

The largest decrease in prices occurred in the Rocky Mountain region, where diesel fell 7.7 cents from the previous week. The smallest decrease was in California, where prices dropped by a negligible 0.2 cents per gallon.

The national average price for regular gasoline actually increased this week, rising 0.8 cents per gallon to $2.034. The price is still 26.5 cents cheaper than it was in the same period last year.

While prices were down in most places, a 6-cent per gallon increase on the West Coast and a 2.7 cent increase in the Midwest drove the national average higher for the week. The largest decrease in prices was in New England-- 2.4 cents.

The price of crude oil increased on Dec. 29, as the stock market prepared for official data on reductions in U.S. oil supply, according to a Wall Street Journal report. While crude oil is still trading near decade lows and is expected to stay there, an official report indicating a decrease in U.S. oil production should keep prices at or above current levels.