Detroit ? General Motors Corp., looking to maintain the momentum of its hugely popular employee-pricing discount plan, has extended the promotion until Sept. 30.

The world’s largest automaker said Thursday the plan now includes select 2006 full-size trucks and sport utility vehicles, including Chevrolet’s Avalanche and Tahoe, the GMC Yukon and the Cadillac Escalade.

“Trucks make up more than 60 percent of our sales,” spokeswoman Brenda Rios said. “We wanted to continue our high summer numbers.”

GM, in June, was the first of Detroit’s Big Three automakers to offer employee prices for all consumers.

Its sales shot up 41 percent that month and more than 19 percent in July. Ford Motor Co. and DaimlerChrysler AG’s Chrysler Group followed with their own employee-pricing plans in July.