U.S. sales of medium- and heavy-duty trucks fell 43.8 percent in May from a year earlier to 16,654 vehicles.
The latest drop pulled sales for the first five months of the year down 38.4 percent to 87,289.
Sales of medium- and heavy-duty trucks have fallen every month since February 2007.
In May, sales fell in every size class, led by a 64.1 percent collapse in Class 5 trucks, to 1,699.
Sales in Class 8, the category for the largest trucks, slid 39.0 percent to 6,955. Class 7 truck sales fell 41.5 percent to 2,887, while sales of Class 6 trucks tumbled 60.0 percent to 1,304. Medium-duty Class 4 trucks skidded 28.6 percent to 3,809.
Ford Motor Co. led all makers as its May sales slipped only 32.8 percent to 3,867. That allowed Ford to pass International, which had been No. 1 in May 2008. International's sales fell 37.4 percent to 3,696. Freightliner was No. 3, down 37.1 percent to 3,380.
GMC posted the largest drop of any truckmaker, as its sales of Class 4-7 trucks cratered 78.5 percent to 518 in May. On June 8, General Motors said it will shut down its medium-duty truck business by July 31 after a four-year search failed to find a buyer. The operations produce the Chevrolet Kodiak and GMC Topkick.