“In this industry, if you’re making 4 percent profits, you’re held in high esteem,” said Jim O’Neal, chairman of the Truckload Carriers Association. “Now we’re looking at the dreaded stagflation: a soft economy and increasing prices on everything, especially fuel.”

Some causes of the run-up are seasonal, said Ron Planting, an economist at the American Petroleum Institute. For instance, home heating oil and diesel are close cousins, and when heating-oil demand goes up in the winter, the prices of both fuels often rise.

But other reasons are relatively new. The rising popularity of diesel vehicles in Europe increases demand for the fuel and decreases demand for gasoline, allowing European refiners to export their surplus to the United States, which helps stabilize gasoline prices here, Mr. Planting said. Diesel use is on the rise in fast-growing countries like India and China, Mr. Kloza said. And even in the United States, demand for diesel and heating oil grew 1.5 percent in 2007, compared with 0.4 percent for gasoline, Mr. Planting said.

Americans burned about 142 billion gallons of gasoline and about 65 billion gallons of diesel and heating oil in 2007, according to data from the Energy Information Administration. Diesel is mostly burned by businesses to power machinery or haul freight, Mr. Planting said. Since few of the functions dependent on diesel are discretionary, companies must pay the higher prices to remain in business.

“In the U.S., gasoline is having its little run,” Mr. Kloza said. “But diesel is much more of a global market. All the signs say that the world’s appetite for diesel is only going up.”

Rising fuel costs come at a difficult time for America’s transport companies, already hit by slumps in new home construction and consumer spending. Truck tonnage fell 1.5 percent in 2007, to just under 11 billion tons, said Bob Costello, chief economist of the American Trucking Association. Rising diesel costs will almost certainly lead to higher consumer prices and more bankruptcies, he said.

Image Tony Jarachovic, waiting for his truck to be unloaded in Aurora, Ill. I have no expenses left to cut, he said. Credit... Peter Wynn Thompson for The New York Times

“We’re already seeing more trucking companies fail, and it’s only going to get worse,” Mr. Costello said.