General Motors Co
The carmaker sold 221,192 vehicles in the month, down from 222,305 in May 2010 excluding four brands the company no longer sells.
The figure fell short of Wall Street and industry forecasts, which had predicted total sales ranging from about 225,000 estimated by data firm TrueCar.com to 242,000 forecast by Jefferies.
U.S. auto sales figures offer one of the first snapshots of consumer demand in May. The rest of the automakers are scheduled to report their May sales later on Wednesday.
GM's U.S. sales chief, Don Johnson, said the annualized industry sales rate for May would be around 12 million to 12.1 million vehicles.
Economists polled by Reuters projected the industrywide annualized sales rate would be around 12.6 million in May, 8.6 percent higher than in May 2010.
But if these forecasts prove true, this will be the first time the annualized sales rate has dropped below 13 million since February. The May sales numbers come as analysts are raising concerns about a slowdown in the broader economy.
GM shares were down 1.8 percent at $31.25 on Wednesday morning.
U.S. private-sector payroll growth slowed sharply in May, falling to the lowest level in eight months and prompting some economists to lower forecasts for job growth in Friday's U.S. government report.
Japanese auto sales fell by a third in May to the lowest total for the month since 1968, as carmakers struggled to restart production after the earthquake and tsunami that roiled the country in March. But South Korean carmakers continued to gain traction, with Hyundai Motor <005380.KS> and affiliate Kia Motors <000270.KS> posting double-digit sales growth, driven by solid demand for new models.
In France and Spain, car sales tumbled as government incentives to replace old models came to an end. French sales fell 8.3 percent on an adjusted basis, while sales in Spain slid 23.3 percent, the 11th straight monthly drop.
(Reporting by Ben Klayman, Deepa Seetharaman and Mike Miller in Detroit, editing by Matthew Lewis)