Ford said May sales rose 2.2% from a year ago to 241,126 units. GM sales dropped 1.3% to 237,364.
GM said it had been trimming sales of heavily discounted vehicles to car rental companies. Such fleet sales made up about 19% of its total sales in May.
Ford's fleet sales rose 8.4%, representing more than 34% of total sales. The industry average is around 20%.
Analysts had expected mixed results for the industry, with sales likely propped up by heavy discounts.
Fiat Chrysler Automobiles said May sales dipped 0.9% to 193,040. Toyota Motor's U.S. sales dropped 3.2% to 156,865. Nissan Motor said U.S. sales in May rose 3.0%, to 137,471.
After demand fell in March and April, analysts estimated May sales at just over 1.5 million. The seasonally adjusted annual rate of sales in May was estimated at 16.8 million to 16.9 million vehicles, about the same as April. A year earlier, sales stood at 17.55 million vehicles.
Early reports indicated that sales over the three-day Memorial Day weekend were helped by heavy discounts.
"While demand for new vehicles is still relatively strong, it's a bit of smoke and mirrors," said Jessica Caldwell, executive director of industry analysis at Edmunds, the car shopping website.
Manufacturers and dealers "really pushed the deals over the holiday weekend to prop up their May numbers," she said. "Incentives were up sharply, and it seems automakers are putting more cash on the hood to nudge car shoppers to buy versus lease."
General Motors dealers were offering discounts of up to $12,000 on the full-size Chevrolet Silverado pickup, while some dealer discounts on Ford Motor Co's F-series pickups were more than $10,000 on 2017 models and more than $14,000 on leftover 2016 models. The 2017 model year started eight months ago.