The turbo-diesel GL320 Bluetec has additional exhaust treatments to cut soot and nitrogen oxide emissions — technology last year's diesel GL320 CDI lacked, which is why California and several Northeastern states wouldn't sell it. The new diesel six-cylinder performs like most diesels of its kind: burly and full of grunt early on, but loathe to produce much more power when you rev high. Indeed, it has more torque than the V-8 GL550, and it puts it all on the plate at just 1,400 rpm. Accessing that power is harder than it sounds, though, as the Bluetec is plagued by a bad case of accelerator lag, and its seven-speed automatic isn't particularly responsive. Put together, it takes a patient right foot to nurse the proper acceleration out of the beast; prod the pedal hard, and the response is tepid at first, but after a second or so it's probably more than you asked for.

Though I didn't drive the V-8 GL450, Mercedes quotes a 7.4-second zero-to-60 mph time, which is impressive for something that weighs nearly as much as a Chevy Tahoe. The GL550 is the lineup's hardest hitter: It uses Mercedes' 5.5-liter V-8, a thoroughly modern engine shared with some of the brand's best models. The engine's wide power band propels the GL to freeway speeds with ease, and the automatic transmission feels better groomed for this application, downshifting two or three gears at a time for quick, confident bursts of power.

Alas, accelerator lag still rears its head. It isn't apparent during stop-and-go driving, but a sudden call for power — escaping a slow-moving freeway lane, for example — is sometimes met with a lazy response. I try to tolerate some lag, as it's often a byproduct of the electronic throttles that most cars, including the GL, employ these days. What frustrates me is inconsistent response, and that's what the GL exhibits. Sometimes the gas pedal is virtually lag-free, while other times it takes half a second or longer to summon the appropriate oomph.

Antilock brakes with discs at all wheels are standard. The pedal delivers strong response, but it's a bit touchy when you lift back off, so smooth stops may take practice.