The Mercedes-Benz GL SUV is big. Three rows of seats big. More than 5,400 pounds big. The wonder of this giant SUV is that once you are chugging along a secondary road with a good head of steam, it somehow doesn鈥檛 feel all that gargantuan. I鈥檓 referring to the once-popular Chevy Suburban and Ford Excursion, land-bound barges which slurped down gasoline by the barrelful. Both were 18 feet-something long. The GL serves the same alleged purpose as those hulks: To haul you and your extended family and their junk. Yet all too often these big SUVs are commandeered by the driver riding alone. Hence today鈥檚 full-size SUVs call for better drivability and increased fuel efficiency. The GL is 16.75 feet long, and seats as many as seven people. Fold down the middle and rear seats and you鈥檒l open up 93.8 cubic feet worth of space. I鈥檝e seen smaller Manhattan studio apartments. The truck鈥檚 bulk is well disguised by a mix of gentle folds and body contours, a sculpted tailgate and a spoiler jutting off the roof.

They trick the eye into reading the sheet metal as separate components rather than an oversized blob. Up front, the shape of the headlights is dynamic, playing off the bluntness of the grill. The hood slopes and drops off at the sides, improving sight lines and giving the driver a good idea of the fenders鈥?width, making it less stressful when backing out of tight garages or navigating cramped side streets. All-wheel-drive is standard. The GL moves through curves with confidence despite a soft, cosseted ride. Like an increasing number of oversized vehicles whose engineers seem to have taken their cue from Range Rover, it has an adjustable air suspension. The ride height changes depending on the situation. It squats at highway speeds and can be raised to crest over obstacles in the road. Still, as countless Suburban owners have learned, fuel efficiency matters, particularly when you鈥檙e standing in front of the gas pump, cringing.

The 362-horsepower GL450 has a 4.6-liter V-8 that gets 14 miles per gallon in town and 19 on the freeway. Not super economical. The more-powerful models, the 429-hp GL550 and the 550-hp GL63 AMG, get even poorer mpg. Potential buyers might consider the diesel model I tested. The GL350 BlueTEC has a 3.0-liter, turbocharged diesel V-6. This alternative powertrain is mated with a seven-speed automatic transmission and manages 19 city, 26 highway. 63,050, making it slightly cheaper than the GL450. Diesel is incredibly popular in Europe, and it鈥檚 heartening to see more diesel models making their way to the U.S. Not long ago there were only a few diesel SUVs available, including the BMW X5, Volkswagen Touareg and Audi Q7. Now Mercedes offers versions of the ML and GL and, soon, the smaller GLK. Porsche has a diesel Cayenne and the Jeep Grand Cherokee will soon offer a diesel V-6. The GL350 makes only 240 horsepower and, characteristic of a diesel, it churns out 455 pound-feet of torque. That means power delivery is very different from a high-revving, high- horsepower engine. Think of it more like a charging polar bear rather than a jaguar鈥檚 high-spirited sprint to top speed.

Poke at the diesel GL鈥檚 accelerator and it takes a moment to react. Once it鈥檚 moving, the full strength and anger are already there. Hence the BlueTEC hustles up even the steepest highway hills while never seeming to labor hard. Revolutions per minute are remarkably low -- max engine speed is 4,750. Stab the gas to make a sudden pass, though, and the moment鈥檚 pause can seem interminable. Drawbacks are few. My GL started up in extremely cold temperatures without any hesitation, and unlike old diesels, doesn鈥檛 spew black smog. Still, every 10,000 miles or so, drivers have to refill the 8.3-gallon urea tank with a fluid, called Adblue, which helps counteract diesel鈥檚 exhaust pollutants. While you can do it yourself, most owners have it done at the dealership, which means added cost. If you do pack the family in, most of your passengers will find the GL plenty roomy, including those in the second row. The third row is harder to access; better for kids than grandparents. 1,090 panoramic sunroof, making the cabin lighter and airier. The GL is definitely the giant on Mercedes鈥檚 lot. Fortunately, it鈥檚 a gentle one.