Industry researcher firm Autodata said Ford sold 246,019 vehicles in the month, including 87,428 cars and 158,591 light trucks.
Ford's truck sales rose 16.8 percent year over year, while its car sales rose 9.4 percent, Autodata said.
Ford's total sales came close to the General Motors sales total for the month of 252,894 vehicles. But sales at GM rose only 3.1 percent from May to May, with a 6.4 percent decline in car sales over the same period to 94,175 and a 9.7 percent gain in light truck sales to 158,719.
GM in the month sold only 128 more light trucks than Ford.
Chrysler sales rose 11 percent with a 15.8 percent gain in car sales to 54,351 and an 8.9 percent gain in light truck sales to 112,245.
Among foreign brands, Nissan North America Inc. sold 114,457 vehicles in the month, a 24.7 percent gain from May 2012. Volkswagen sales were up 2.2 percent from May 2012 with 51,499 vehicles sold.
Hyundai, Honda and Toyota sales numbers were not included in the report.