The Sierra HD and its twin the Chevrolet Silverado HD are designed for heavy towing and hauling. The 6.0-liter V8 and 6.6-liter turbodiesel are both mated to a six-speed automatic. The latter works well, and employs a heavy-duty Allison-made transmission. We measured 14 mpg overall with the diesel. The LTZ trim has a luxurious cabin but ascending to into it is quite a climb. If you don't plan to go off-road, the Z71 suspension is superfluous -- it makes the ride extremely stiff, bucking uncomfortably on every bump. The cabin is quiet but the diesel clatter is audible at low speeds. Handling is clumsy but ultimately secure. The HD pickups are the better choice for commercial purposes and lend themselves to snow-plowing duties.