General Motors Co.’s sales fell about 13 percent in August as the automaker pulled back on sales incentives, especially for full-size pickups, people familiar with the matter said.

The total sales result, which GM no longer reports every month to the public, missed analysts’ average estimate for a decline of 7.7 percent.

Jim Cain, a company spokesman, declined to comment on the sales drop, though he did confirm that GM dialed back discounts during the month.

GM shares extended declines, dropping 1.3 percent to $35.58 as of 2:32 p.m. in New York trading. The stock is down 13 percent this year.

With the exception of Ford Motor Co., all other major automakers also reported sales that trailed analysts’ estimates, as demand for passenger cars including the Honda Accord and Toyota Camry plunged.