Rising fuel prices have hit used-car prices, especially for bigger vehicles
General Motors has reported a net three-month loss of $15.5bn (£7.8bn) as North American sales fell by 20%.
GM took a $3.3bn charge for buying out the contracts of 19,000 hourly workers who left at the end of June.
It also wrote off $1.3bn because of reduced values of big, used cars, which cut the value of formerly leased cars owned by its financing unit, GMAC.
Without one-off charges, GM lost $6.3bn compared with a net profit of $891m in the same period of 2007.
It is the third biggest quarterly loss in the carmaker's history.
GM shares fell 4.7% in early trading.
More staff may go
On 15 July, GM announced the latest stage of its restructuring plans, which include laying off thousands of workers, speeding up the closure of truck and sports utility vehicle (SUV) plants, selling assets and suspending its dividend.
On Friday, the carmaker said it might offer another round of buyout and early retirement offers to its 74,000 workers in the US.
About a quarter of its US hourly workforce took the offers in the last round, which ended on 1 July.
The staff cuts are needed because GM is cutting production as a result of falling sales of vehicles, especially SUVs and trucks.
GM is not the only company suffering from the state of the car market.
Other firms suffering
Earlier in the day, BMW warned that its profits for 2008 would be below forecasts and predicted a "difficult" 2009.
Also on Friday, Nissan reported a 42.8% fall in its three month profits.
Net profit fell to 52.80bn yen ($491m; £248m) between April and June, compared with 92.31bn in the same period of 2007.
"In the face of the severe operating environment, Nissan remains resilient but cautious on the outlook for our industry," said chief executive Carlos Ghosn, who also runs Nissan's French partner Renault.
Nissan said it was trying to limit the effects of the slowing US car market by raising prices, cutting jobs and reducing truck production.
Toyota and Ford also unveiled their July US sales figures.
Ford said its car sales under the Ford, Lincoln and Mercury marques were 8% higher than July 2007.
Its Focus car was the star performer, with sales up 16% in July compared to a year earlier.
However, sales of SUVs were 54% lower and trucks and vans were down 18%.
Toyota also saw a large fall in sales of its larger vehicles in the US.
It sold 32% fewer SUVs than in July 2007 and 33% fewer light trucks. Passenger car sales were also 8% down.