Allison 1000 6-Speed Automatic and 4WD. Handles off-road like a real dirt devil. Sizeable room overhead and underfoot. Stop clicking the mouse because this 2015 GMC Sierra 2500HD is the truck you've been looking to get your hands on. You, out on the road in this outstanding, one-owner Sierra 2500HD, would look so much better than it sitting here, all sad and lonely, on our lot. It's only known one loyal master, but now it's ready to be loved again. Come get it! Expect A Great Deal!