The Mercedes-Benz GL550 can accelerate from 0 to 60 mph in 6.4 seconds, according to Mercedes-Benz USA.   The 382-hp V8, with four valves per cylinder, generates 391 lb-ft of torque from 2800-4800 rpm with all the smoothness you'd expect in a flagship.   It also generates EPA ratings of 12/17 mpg.  

The GL450 is more than adequate with its 335-hp (339 lb-ft; EPA 13/17) 4.6-liter V8 if towing or speed contests aren't on your agenda; Mercedes says only a half-second longer than the GL550 (and we think their estimates are conservative).   Apart from the 21-inch wheels and more aggressive bodywork, a lesser GL-Class can be optioned to match a GL550 in features.   For poor roads, winter climes that require snow chains or anyone inclined to use the low-range gear in the Off-Road package, we think the GL450 with its 19-inch wheels is a better choice than either the GL550 with its 21-inch wheels or the GL350 with its 20-inch wheels.  

The GL350 uses diesel fuel rather than gasoline.   The GL350's BlueTEC turbodiesel 3-liter V6 delivers 210 hp.   More important is its 400 lb-ft of torque from 1600 rpm upwards.   While any engine is affected by altitude, the turbodiesel will lose a smaller percentage of its power as elevation rises.   The GL350 will take about two seconds longer to reach 60 mph, which it does right in line with most seven-seat hybrid SUVs.   Diesels don't snap your neck from a standing start but once off the line the GL350 BlueTEC accelerates with more than adequate verve.   The GL350 is also clean.   The exhaust is actually cleaner than the ambient air in many smoggy areas.   It rates an EPA-estimated 17/23 mpg.   Our test drives have found EPA diesel ratings a bit pessimistic as we averaged better than 24 mpg in myriad conditions.   Every GL-Class may be equipped to tow 7500 pounds, giving the high-mileage diesel a substantial advantage over hybrids in this respect. A 7-speed automatic transmission is standard on all GL-Class models and operates seamlessly as an automatic.   On the other hand, you can shift it yourself with paddles on the steering wheel.  

The four-wheel-drive system, called 4MATIC, uses open front, center and rear differentials.   Quite sophisticated, 4MATIC is designed to maintain mobility even when only wheel has traction by electronic intervention.   In normal driving, the system distributes power equally to front and rear wheels.   The system includes DSR, a sort of low-speed cruise control for regulating downhill speed, and Hill-Start Assist, which keeps the GL-Class from rolling backward when launched after being stopped on an 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, and with the GL550's adaptive damping 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, and it levels the car regardless of load for stable handling, towing and visibility.  

We've driven all three GL-Class models on freeways, back roads, fire roads, and trails marked for off-highway vehicles.   Steering feedback was consistent, brakes (14.7-inches in diameter on the front wheels and 13.0 inches on the rear) were responsive and consistently predictable and the GL-Class offered nice balanced dynamic capabilities when hustled through the curves on the narrow, hilly roads that wind through the vine-covered hillsides.  

The GL550's fat 21-inch tires contribute commendable grip for hard cornering.   However, these wide, low-profile tires follow pavement grooves, give sharper impact sensations, have notably higher replacement costs and can not use tire chains.