The 2500HD Classic is a three-quarter-ton full-size truck. As a regular cab model it can seat up to three passengers. Extended cab models can carry up to six passengers. It is powered by a 300 hp 6.0-liter V-8 engine. There is a choice between a five-speed manual and a four-speed automatic.The GMC Sierra 2500HD Classic is a carryover from 2006.