Allison 1000 6-Speed Automatic, 4WD, Ebony Cloth. 2013 GMC Sierra 2500HD SLE 4WD Duramax 6.6L V8 TurbodieselReviews: * Impressive hauling and towing capabilities, refined and quiet ride, solid build quality, responsive steering. Source: Edmunds * If you require a vehicle that can transport over 10,000 pounds without breaking a sweat, the 2013 GMC Sierra HD deserves a spot on your short list. Furthermore, most prospective HD truck buyers who place a premium on luxury will find the Sierra HD Denali worth springing for. Source: