LOS ANGELES — General Motors, Ford Motor Co. and Chrysler LLC may drag U.S. auto sales for October to their lowest levels since 1992, a forecasting firm said last week. Overall industry volume will plunge 29 percent to 872,000, Edmunds.com said. GM will suffer most with a 41 percent decline for its domestic brands, Edmunds said. Ford and Chrysler will be down more than 34 percent. Among the biggest Japanese automakers, Nissan's volume will fall 29 percent, Honda will drop 17 percent, and Toyota will slide 16 percent, Edmunds said.