6-Speed Automatic with Overdrive and 4WD. What a fantastic deal! Wow! What a sweetheart! If you travel a lot. you're going to LOVE this terrific 2012 GMC Sierra 3500HD with low. low mileage. While 99% of the time you'll probably be driving on pavement. the 4WD power delivery means you don't have to live that other 1% with regret....