The Z71 package on a Chevy Silverado 2500HD is an off-road package that includes a variety of features, such as: Suspension: Twin-tube Rancho shocks for better suspension on rough terrain Tires: All-terrain tires, such as 18-inch Hankook Dynapro MT2 tires Transfer case: Autotrac two-speed transfer case that provides more power at lower speeds and in low gear Locking rear differential: Provides more power to the rear wheels for getting out of tough spots Hill descent control: Helps you safely come down hills, even when carrying heavy loads Underbody shield and brushed metal sill plates: Protect the truck from bumps Exterior badges and side steps: Black exterior badges and side steps Body-color bumpers and grille: Body-color bumpers and grille Spray and bed liner: Comes with the package Off-road mode: Optimizes the powertrain for off-road conditions All-weather floor liners: Textured, anti-slip liners that replace the factory floor mats