Allison 1000 6-Speed Automatic 4WD Ebony Leather. 2012 GMC Sierra 2500HD SLTSteel Gray Metallic 2012 GMC Sierra 2500HD SLT 4WD Allison 1000 6-Speed Automatic Duramax 6.6L V8 TurbodieselCARFAX One-Owner.Odometer is 48508 miles below market average!Reviews: * Impressive hauling and towing capabilities; refined and quiet ride; solid build quality; responsive steering; comfortable seats. Source: Edmunds * If you’re a GMC loyalist the 2012 GMC Sierra HD truck line is bound to keep you coming back for more. Those who worry about dirtying up the inside of their truck will appreciate the heavily vinyl trappings of the Work Truck line. Source: KBB.com