Consumers are delaying big-ticket purchases because they're worried about their jobs in an environment of high unemployment. Analysts predict overall sales for the industry will drop 10 percent or more from May.
GM was the first automaker to report U.S. sales for June on Thursday.
There were glimmers of strength in GM's results. Sales for the automaker's four core brands - Chevrolet, Buick, GMC and Cadillac - rose 36 percent over June of last year, helped by strong demand for crossovers and some recovery in pickups.
The company's overall sales, which include brands being sold or phased out, increased 11 percent from a year earlier. Still, last June was a relatively weak month as GM headed into bankruptcy protection.
GM is phasing out or selling Saab, Saturn, Pontiac and Hummer.
The industry sales drop from May could be good news for buyers later in the summer. Analysts say if the month-to-month declines continue, automakers will be tempted to expand sales promotions such as low-cost leases, zero-percent financing and cash rebates.
Slow retail sales early in June accelerated in the second half of the month, according to data collected from 8,900 dealers nationwide by J.D. Power and Associates.