The GL550 can accelerate from 0 to 60 miles per hour in six seconds or less. Its 382-hp V8 with four valves per cylinder, is part of a new family of V8 engines from Mercedes-Benz, all designed with a broad torque spectrum for hauling around 2.5 to 3.5 tons of truck.

The GL450 is more than adequate with its 335-hp V8 if towing or speed contests aren't on your agenda. Apart from the 21-inch wheels and more aggressive bodywork, a GL450 can be optioned to match a GL550 in features. Indeed, for poor road areas, severe winter climes that frequently require snow chains, or those inclined to use the low-range gear in the Off-Road package, the GL450 is the better choice.

The GL320 CDI delivers more than adequate thrust from its little V6 turbodiesel engine. In fact, it boasts more torque than the GL550's hefty V8, torque being that force that gets you going and powers you up hills. The GL320 offers slightly superior fuel economy to that of the GL550. If high-altitude driving is routine, it's worth noting the turbocharged diesel engine loses far less of its power than the normally aspirated gasoline engines as the air gets thinner. The diesel is flexible, providing maximum torque from 2800 rpm all the way up to 4800, and horsepower that pulls past 6,000 rpm.

The seven-speed automatic transmission helps keep the GL-Class engines operating in the sweet part of a power band.

The four-wheel-drive system, called 4MATIC, features front, center and rear differentials. Quite sophisticated, 4MATIC is designed to maintain mobility even when only wheel has traction. In normal driving, the system distributes power equally to front and rear wheels. The system includes DSR, a sort of cruise control for regulating downhill speed, and Hill-Start Assist, which keeps the GL from rolling backward when launched after being stopped on uphill slope. 4MATIC also adjusts anti-lock brake controls to provide quicker stops on slippery and unpaved surfaces, an important benefit many vehicles with ABS do not offer.

Airmatic is an air suspension system that uses air bladders instead of coil springs to adjust ride height by as much as three inches as well as ride firmness and body control parameters, so the driver can select from comfort and more aggressively sporty settings. The system automatically lowers the vehicle to enhance handling and aerodynamic efficiency at speeds of 77 mph or higher. Adaptive Damping II, which provides electronic control and driver-selection of shock settings for the best combination of ride comfort and handling response, is standard on the 550, optional on the 450, and not available on the 320.

Those serious about venturing far from pavement can opt for a special off-road package on the GL320 and 450 that includes special wheels and tires, a two-speed transfer case, locking center and rear differentials, adaptive damping and with ground clearance height-adjustment options to allow as much as 12.4 inches of ground clearance compared to the standard highest clearance of about 10 inches.

We did our driving on freeways in and out of San Francisco's airport and on roads through Napa Valley wine country. Steering feedback was consistent, brakes (14-inches in diameter on the front wheels and 13 inches on the rear) were responsive and consistently predictable and the GL offered nice balanced dynamic capabilities when hustled through the curves on the narrow, hilly roads that wind through the vine-covered hillsides. The fat 21-inch tires contribute commendable grip, and like the majority of wide, low-profile tires, follow pavement grooves, give sharper impact sensations, and can not use tire chains.