GMC’s one-ton, full-size pickup is designated the Sierra 3500HD, or Heavy-Duty, and offers several upgrades over the three-quarter-ton 2500, including a beefier rear axle and the option for a dual rear wheel or “dualie” setup. Buyers can choose from Regular, Extended, and Crew cabs, all available with single or dual rear wheel setup, although only the 8-foot-long bed is offered. Regular Cabs, with their dual bucket or 40/20/40-split bench, are available only with four-wheel drive (4WD). Extended Cabs have rear-hinged rear doors that open only when the front doors are ajar, and Crew Cabs get independently opening rear doors. Both get a 60/40-split rear bench and are available with either rear-wheel drive (RWD) or four-wheel drive (4WD). A 6.0-liter Vortec V8 stands as the base engine, producing 360 hp and 380 lb-ft of torque at 5,400 and 4,400 rpm. Isuzu developed a biodiesel-capable, turbodiesel 6.6-liter V8, which they’ve labeled the Duramax. 365 hp at 3,200 rpm is a mild upgrade over the 6.0, but 660 lb-ft of torque at 1,600 rpm is an incredible figure. Both engines come with six-speed automatic transmissions with tow/haul modes, but the Duramax gets the Allison 1000 six-speed, with its electronic engine grade braking. Standard external transmission and engine-oil coolers, as well as a 3.73 rear axle, are included, with a 4.10 rear axle available for the 6.0-liter engine. Maximum towing capacity is 13,000 pounds with an external hitch and 16,500 pounds with the in-bed 5th-wheel hitch and dual rear wheel setup, which GM claims are both best-in-class figures. There are actually two 4WD systems to choose from, and while both have a low-range gear, one is a part-time system for inclement conditions, and the other is a full-time system that can be left engaged on dry pavement. Both come with automatically locking center differentials and a heavy-duty suspension. With Pure Pickup and Luxury Inspired interior designs, additional customization can be made over the basic three trim choices of Work Truck (WT), SLE, and SLT. Choosing a WT with the Pure Pickup interior offers a utility-geared design that some testers find leans too heavily on hard plastics and a Spartan ideal. An SLT with a Luxury Inspired interior offers luxury options that are missing on many cars, such as heated seats, touch-screen navigation, rear-seat DVD entertainment, and heated windshield-washer fluid. There’s also a Z71 off-road package with the Z71 suspension and a full skid plate package.