This massive GM, set for the stage at the 2011 Detroit Auto Show, features a 6.6-liter 397 horsepower Duramax turbo-diesel V8 with 765 pound-feet of torque. The transmission is a six-speed Allison 1000 unit. The Sierra All Terrain concept is shorter than a traditional HD Sierra, but features upgraded suspension with three-inches more clearance, a wider -- 73 inch -- track, front and rear jounce shocks, electronic disconnect stabilizer bars, composite underbody protection, 35" BF Goodrich tires on 20" rims and Fox shocks with remote reservoirs in the wheel liner.