Overall Ford sales in the United States dropped by 7.5 percent in July to 200,212 units, mostly because of a decline in fleet sales.
Shares of most major automakers have trailed benchmark USA stock indexes this year. Separately, rival General Motors Co.'s stock slid 3% after reporting July vehicle sales fell 14%.
Several of the big automakers, including Ford, GM, Fiat Chrysler Automobiles, Nissan Motor Co. and Hyundai Motor Co. announced they sharply reduced sales of rental cars during July, and portrayed those choices as placing profit in front of sales volume.
One bright spot for all the automakers: auto shopping site Edmunds.com reports the average price paid for a new vehicle in July rose two percent to $34,558.
This year, big cuts in sales to rental auto fleets and commercial customers were also a factor. With labor agreements being more flexible, the automakers in Detroit have shifted away from that. Only the Ford F-series, up 6 percent, improved on year-ago results among major truck models. The drop was lower than the 7 percent decline in June.
Declining sales could leave the Detroit Three with too many plants in North America, Jeff Schuster, a forecaster with LMC Automotive, said on Tuesday at an industry conference in Traverse City, Mich.
GM did not gain much ground trying to reduce the inventories of its unsold vehicles.
Fiat Chrysler posted a 3 percent drop in monthly sales in Canada, as its Dodge and Chrysler brands weighed on sales.
Canada's economy grew by 0.6 percent in May from a month earlier, Statistics Canada said last month, exceeding economists' forecasts for 0.2 percent.
Retail sales fell by only 1%, with Ford's share of the retail market actually increasing over the last three months inluding July.