The nation's top automakers registered mixed sales numbers in May according to their latest monthly reports.

Ford was the only company of Detroit's "Big Three" to report a sales increase compared to May 2016. The Dearborn, Mich., automaker saw sales climb by 2 percent, while sales for General Motors and Fiat Chrysler declined by about 1 percent.

The top overseas automaker — Toyota — also saw a slight sales decline last month as Japanese rivals Honda and Nissan reported increases of 1 percent and 3 percent, respectively.

Those six companies accounted for more than 75 percent of the U.S. auto market in April, according to The Wall Street Journal.

(AP Photo)

Last month's numbers were helped by one additional sales day compared to May 2016, but companies also touted numbers for larger vehicles, which continued to sell at robust levels — but often at the expense of smaller cars.

GM said that retail sales of its crossovers jumped 19 percent over that span, while FCA's Ram pickups increased by 8 percent. Ford reported the strongest F-Series pickup sales in more than a decade and a record month for SUV sales.

Nissan, which reported the strongest gain among the top six automakers, said that overall truck, crossover and SUV sales climbed by 22 percent. The Armada full-size SUV and Titan pickup increased their sales by more than 200 percent and 400 percent, respectively.

Analysts believe that 2017's sales could fall short of the record levels seen in 2016, and some are concerned that rising inventories could lead to steeper dealership discounts — and lower automaker profits.