Diesel Power

The GL350 Bluetec's 3.0-liter diesel V-6 makes 210 horsepower. There's some familiar diesel clatter when accelerating, though the noise is nothing like an 18-wheeler. The sound is more akin to that of the newer diesel pickup trucks from Ford and Chevrolet, but it's not as inconspicuous as today's quietest clean-diesel cars.

If you're considering the GL350 over a gas-powered GL, it may be because you've heard that diesels produce a lot of low-end power, or torque, providing spirited acceleration from a stop. This one does — the engine makes 400 pounds-feet of torque at just 1,600 rpm — but the delivery isn't as satisfying as in some diesel cars, like the Volkswagen Golf TDI. The Golf TDI wallops you when accelerating, pushing you back in your seat. By comparison, the GL350 feels burdened by its hefty 5,423-pound curb weight. It gets around well enough, but it doesn't move out authoritatively.

Diesels also typically hold a fuel economy advantage over gas engines. The GL350's combined EPA rating of 19 mpg is about 27 percent better than the 15 mpg combined rating the GL450 gets with its 4.6-liter gas V-8. However, the GL350's combined rating lags those of other large diesel SUVs, including the BMW X5 xDrive35d (22 mpg) and Audi Q7 TDI (20 mpg).

That said, in everyday driving it's easy to achieve average fuel economy figures that are higher than the EPA estimates. An hour-long drive on highways and suburban roads around Chicago returned average fuel economy of about 25 mpg. With a large 26.4-gallon fuel tank, the cruising range could easily stretch to 600 miles, which is good for both road trips and the inevitable day you have to hunt for a station that sells diesel.

The engine teams with a seven-speed automatic transmission. There are manual-shift paddles on the steering wheel, but they seem even more out of place in the GL than they do in most vehicles. They do, however, provide a way to hold a gear on steep grades that wouldn't otherwise be available, as the SUV uses Mercedes' stalk-style gear selector rather than a traditional shifter. The only real downside to the transmission during my test was that on two occasions, just after getting under way, it clunked when shifting at moderate speeds and light throttle. Hardly the kind of thing you'd expect from a Mercedes.

Mercedes cars don't always have the best brake-pedal feel, but it feels pretty natural in the GL350. Stopping response is very linear, and the brake pedal has deep travel that lets you finely tune braking response.

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