General Motors Co. said its US sales rose nearly 15 percent in August on big gains in full-size pickup trucks and tiny cars. GM sold almost 276,000 autos — its best month since September 2008. Toyota, Ford, Nissan, Honda, and Chrysler also posted double-digit gains. Sales of GM’s Chevy Silverado pickup were up nearly 14 percent. Cadillac sales were up nearly 38 percent; Buick sales were up 37 percent. Standard & Poor’s Ratings Services boosted GM’s outlook to positive from stable.