General Motors shares ended 4.4% down, while Ford  dropped 4.3% and US traded shares of Fiat Chrysler lost 4%.
Car sales over the last year and a half have soared due to pent-up demand from the recession.
But as the market has become more saturated vehicle sales are slowing, a trend predicted last week by Ford when it announced its earnings.
Sales of Ford were down 3% from last year to 216,479 vehicles, according to the company.
GM sales declined by 1% to 267,258 vehicles.
Fiat Chrysler was the only one to report an uptick in sales, but the rise was just 0.3%. The Italian-American carmaker is under investigation by the US Justice Department for the way it records sales and last month restated sales figures dating back to 2011.
One continuing trend in the US has been the popularity of larger and more expensive sports utility vehicles and trucks.
These sales help lift profits of carmakers like GM even as overall sales fall.
Shares of US car makers fell more than 4% after data for July sales came in below analysts expectations.