The price of on-highway diesel fuel and regular gasoline dropped slightly in the last week of 2015, capping off a year marked by falling energy prices, according to recent Energy Department numbers.

The average price of a gallon of on-highway diesel fuel dropped 2.4 cents from the previous week, settling at $2.211. The price ended the year 92.6 cents cheaper than for the same week in 2014.

Prices were down across the U.S., with the largest decrease in prices coming to the Rocky Mountain region at 3.6 cents. The smallest decrease in prices was a miniscule 0.3-cent decrease on the West Coast.

The average price of regular gasoline was essentially flat for the week with a 0.6-cent drop to $2.028 per gallon. That price is 18.6 cents cheaper than in the last week of 2014.

Prices were down in most areas, except for the West Coast where gas prices increased by 3.3 cents. The area that saw the largest decrease in prices was the Central Atlantic at 1.9 cents.

The price of crude oil was down on Jan. 4, after an up-and-down day of trading due to a possible conflict between Saudi Arabia and Iran, according to a MarketWatch report.

Market instability was expected by some oil analysts to start 2016 as tensions increased between two of the most influential countries in OPEC after Saudi Arabia severed ties with Iran.

Big test Monday: Saudi/Iran hostilities, plenty of big money shorts, and influx of 2016 money sets up possible temporary rally for oil.

— Tom Kloza (@TomKloza) January 3, 2016

Saudi Arabia executed a prominent Shiite cleric, which sparked outrage in the predominantly Shiite population of Iran. Despite the tensions, oil prices are expected to remain low-- barring an actual war between the two countries.