Shortly after last year's GL450 arrived, Mercedes introduced a diesel GL320 CDI; click here to compare the 2008 GL-Class with the 2007 model. Thanks to the higher efficiency of its diesel engine, the CDI's mileage is considerably better than its V-8 gas siblings, but because of stricter emissions standards the diesel isn't available in California and several northeastern states. (For 2009, a cleaner GL320 Bluetec version will be available nationwide.) All GL-Class models include full-time four-wheel drive. Here's how the engines compare:
Engines Compared
GL320 CDI GL450 GL550
Engine 3.0-liter V-6 4.6-liter V-8 5.5-liter V-8
Horsepower (@ rpm) 215 @ 3,800 335 @ 6,000 382 @ 6,000
Torque (lbs.-ft. @ rpm) 398 @ 1,600 — 2,800 339 @ 2,700 — 5,000 391 @ 2,800 — 4,800
Gas mileage (mpg, city/hwy.) 18/24 13/18 13/17
Required fuel Diesel Premium gas Premium gas
Source: Automaker data for 2008 models.

The GL550's 5.5-liter V-8 sees duty across a number of other Mercedes models. It's a powerful engine, mustering enough torque to propel the SUV to freeway speeds with ease. A seven-speed automatic is the standard transmission for all engines. Gear hunting is minimal, and downshifts can come two or three gears at a time — from sixth to third, for example. The resulting shorter ratios for each gear allow quick, confident bursts of power.

Unfortunately, starting out can be troublesome. A few days into my test drive, I began to notice some significant accelerator lag. It wasn't apparent during stop-and-go driving, but a sudden need for power — escaping from a slow-moving freeway lane, for example — was sometimes met with a lazy response. I try to tolerate some accelerator lag, as it's largely 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 this SUV exhibits. Sometimes the gas pedal proved virtually lag-free, while other times it took a half-second or longer to summon the appropriate acceleration. Mercedes spokesman Rob Moran said the way Mercedes tunes its accelerators — which generally require more pressure than a typical car's — might have something to do with it.

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.