The GL-Class is the biggest, bluntest, and heaviest sport-utility vehicle Mercedes-Benz offers in the US. For 2009, it can be ordered with a 50-state clean diesel engine that improves fuel mileage and actually costs less than the gas-guzzling V8 alternative. The MSRP is $58,200, with an $1,800 tax credit to support “advanced lean-burn vehicles.”

Of its three clean diesel sport utilities, the Mercedes ML320 Bluetec is the standard suburban five-seater, and the six-seat Mercedes R320 Bluetec is for those who want something closer to a traditional station wagon. But the big GL tends toward brawn, even bling—like the low-volume original G-Class.

How Does Benz Say Clean Diesel? “BlueTec”

For 2009, the GL and its two stable mates are offered with a 210-horsepower, 3.0-liter turbocharged, direct-injection V6 diesel—as well as a 4.6-liter V8 engine. As the world’s first carmaker to offer a passenger diesel—way back in 1936—Mercedes-Benz is determined to offer diesels in many of its US vehicles despite their historic status as niche vehicles.

The 3.0-liter turbodiesel uses the company’s “BlueTec” suite of pollution control devices, including particulate filters and two catalytic converters. It also injects liquid urea into the exhaust stream, using the chemistry to cut oxides of nitrogen (NOx) emissions, which along with particulates form the Achille’s Heel of modern diesel engines.

The diesel gets respectable mileage for an 18-foot-long vehicle that weighs 5,300 pounds. The GL320 BlueTec is rated by the EPA at 17 mpg city / 23 mpg highway, an improvement of 24 percent and 22 percent respectively against the V8 model’s 13 mpg city / 18 mpg highway. As with many new diesels, real-world mileage may be higher than the EPA numbers. Automobile magazine achieved an indicated 26 mpg in energetic driving during their test of a diesel GL.

Unlike Benz’s previous 320 CDI diesels, the BlueTec models are offered in all 50 states, easily meeting California’s stringent Tier 2, Bin 5 emissions standards. But unlike the other two, the GL320 BlueTec is actually cheaper—by roughly $2,500—than the V8 version with lower mileage.

Big and Blingy

As the heaviest of Benz’s three SUVs, the GL 320 BlueTec isn’t particularly fast, taking 9.0 seconds to accelerate from 0 to 60 miles per hour. But what it lacks in speed, it makes up for in brute strength and capacity. The GL offers three-row seating and, with a hair under 400 foot-pounds of torque, the diesel version can tow up to 7,200 pounds.

Those capabilities make it direct competition for the largest and most luxurious domestic SUVs, the Cadillac Escalade and Lincoln Navigator, as well as Audi’s Q7. (Each of the US trucks offers a version with an extended wheelbase, however, a model the GL forgoes.) But perhaps a greener direct comparison would be to the Cadillac Escalade Hybrid, which returns 20 mpg city / 21 mpg highway.

Mechanically, the GL320 BlueTec comes with the superlative Benz 7-speed automatic, mated to standard all-wheel-drive. The height-adjustable suspension adds up to 3 inches of ground clearance, and hill-descent control is standard. It comes with no fewer than nine airbags, as well as the Pre-Safe system that anticipates accidents and pre-loads the brakes and seatbelts, among other safety features.

But it’s the luxury features that make this big Benz more than just a truck. While leather upholstery is optional, standard electronic features include a media control system with a 6.5-inch hi-def display, Bluetooth integration, and a DVD player. The lengthy options list starts with a rear-seat DVD system, adaptive damping, three-zone climate control, and a premium package that bundles a rear-view camera, voice-controlled navigation, and HD Radio with Sirius Real-Time Traffic.

Adding AdBlue

The “AdBlue” liquid urea solution is held in its own tank, which must be refilled about every 12,000 miles. To comply with EPA mandates that a vehicle’s emissions system must work properly for 8 years or 100,000 miles without any owner intervention or maintenance, Benz includes AdBlue refills in mandatory dealer service intervals every 12,000 miles.

What happens if the owner runs late, or the car runs low? After a series of warnings, the BlueTec diesel simply refuses to start if its emissions system doesn’t have enough AdBlue, meaning the car must travel back to the dealer via flatbed truck.