Updated from 12:50 p.m. EST

Detroit's Big Three automakers posted another round of sales declines for December, while Toyota ( TM) continued to gain ground.

General Motors ( GM) said Wednesday that its U.S. sales for the month dropped 9.6%, and it lowered its first-quarter 2007 production forecast to 1.120 million vehicles, down 20,000 vehicles from its previous target.

The world's largest automaker sold 341,327 vehicles in December, compared to 392,041 vehicles in the same month last year. Truck sales dropped 15.5% while car sales were up 2.2%.

The No. 2 U.S. automaker, Ford ( F), said December U.S. sales slid 13% from a year earlier, hit by big drops in sales of its F-Series trucks and discontinued Taurus and Freestar brands.

Ford sold 233,621 vehicles in December, compared with 267,881 a year earlier. Truck sales sank 14% to 163,003 units, while car sales fell 9.9% to 70,618 units.

The declines cap off a year in which GM's vehicle sales fell 9% while Ford's were down 8%, as higher gas prices, a big housing downturn and a shift in demand toward more energy-efficient vehicles hurt truck sales.

Ford's sales of trucks fell 14% for the year to 2.9 million vehicles, and the company said it expects the gas and housing issues to weigh on the segment in 2007. Ford's car sales, meanwhile, rose 5% in 2006.