Diesel prices have dropped almost 50% since record levels last year, but fleets continue to work on cutting fuel costs, one of their largest expenses.

U.S. retail diesel prices dropped 4.6 cents last week to $2.496 a gallon, the Department of Energy reported — the fourth straight decline — but rising crude oil costs will soon lead to a reversal of that trend, a department analyst predicted.

The average cost of retail diesel in the United States dropped 5.2 cents a gallon last week to $2.542, the third consecutive decline and a level nearly 50% below the record highs of a year ago.

The European Union last week imposed tariffs on biodiesel imported from the United States to counter low prices it said were caused by subsidies, but a spokesman for the U.S. Trade Representative denied subsidies were the main reason for the growth of U.S. sales in Europe and that the United States would review the tariffs “closely.”

U.S. retail diesel prices slipped 0.8 cent last week, providing a breather for the trucking industry after seven consecutive weeks of increases that had pushed the average cost to a seven-month high.

The U.S. average diesel price rose 4.4 cents a gallon last week to $2.616, the seventh straight weekly increase that has pushed it up by 43.1 cents to the highest level in nearly eight months.