Diesel prices across the United States hit the highest national average since December 2014 during the week ending Sept. 27, according to the Department of Energy’s weekly report.

The U.S.’ average price for a gallon of on-highway diesel now stands at $3.406, up 2.1 cents over the previous week. This week’s price is the highest since the week ending Dec. 15, 2014, when prices averaged $3.419 per gallon nationwide.

Fuel prices have been steadily increasing since near the end of 2020. Prices fell to $2.372 during the week ending Nov. 2, 2020, and have since increased by more than a dollar.

Last week the most significant increase was seen in the Midwest region, where prices jumped by 3.6 cents. Only two regions – the Rocky Mountain and New England regions – saw a decrease during the same week, with prices falling by 2.4 cents in the Rocky Mountain region and by a tenth of a cent in New England.

The nation’s most expensive diesel can be found in California at $4.339 per gallon, followed by the West Coast less California region at $3.664 per gallon.

The cheapest fuel can be found in the Gulf Coast region at $3.142 per gallon, followed by the Lower Atlantic region at $3.288 per gallon.

Prices in other regions, according to DOE, are:

New England – $3.299
Central Atlantic – $3.514
Midwest – $3.326
Rocky Mountain – $3.605
ProMiles’ numbers during the same week saw fuel prices increase by 1.7 cents, bringing its national average to $3.314 per gallon.

According to ProMiles’ Fuel Surcharge Index, the most expensive diesel can be found in California at $4.282 per gallon, and the cheapest can be found in the Gulf Coast region at $3.088 per gallon.