Carmakers have used lower-margin sales to rental fleets as a way to avoid shutdowns in plants.
In early reporting Tuesday, General Motors Co. sales were down 15.4 percent from a year ago, Ford Motor Co. said its sales were down 7.5 percent and Fiat Chrysler Automobiles' sales were down by 10.5 percent.
After seven years of USA auto sales growth, this year's persistent slump - and anxiety over upheavals from electrification, autonomy and mobility services - has investors shunning automakers as benchmark stock indexes graze record highs.
"Customers across the country drove retail gains of Ford brand SUVs in July", U.S. Marketing, Sales and Services Vice President Mark LaNeve said.
Fiat Chrysler's Ram truck division had flat sales, but Jeep fell 12.3 percent, Chrysler fell 30.1 percent, Dodge was off by 11.9 percent and Fiat lost 18 percent. New vehicle sales hit a record 17.55 million a year ago.
Compared to this time last year, "we're starting off at higher incentive levels; we've got even more elevated inventory levels and even worse inventory-quality problems" heading into the end of the model year, he said.
Mark LaNeve, Ford Motor Co.'s US sales chief, said automakers have been preparing for lower sales this year.
GM gained little ground in its effort to reduce inventories of unsold vehicles. Ford shares were down three percent, and Fiat Chrysler was down 0.025 percent.
Ford Motor Company's stock price fell almost 3% following the release of its July sales report. Auto shopping site Edmunds said the average price paid for a new vehicle in July was $34,558, 2 percent higher than the same month a year ago. Ford Motor Co. was down about 2.5%, while General Motors Co. fell by 3.5%.
But vehicle sales are plummeting, hurt by low gas prices and changing tastes.