Thanks to diesel engines' higher efficiency, the Bluetec achieves nearly the same EPA ratings as a Cadillac Escalade Hybrid. The figures are 27 percent better overall than those of the gas GL450, though the latter model has a V-8 versus the Bluetec's V-6, so this isn't necessarily an apples-to-apples comparison. This year's Bluetec is also 1 mpg worse than last year's GL320 CDI. Mercedes spokesman Rob Moran said that's due to this year's larger wheels — 20-inchers versus 18s, which introduce "quite a bit more rolling resistance." In real-world driving, Moran claims the differences are slighter than the EPA ratings suggest. Here's how the engines compare:
Engines Compared
GL320 Bluetec GL450 GL550
Engine 3.0-liter V-6 4.7-liter V-8 5.5-liter V-8
Horsepower (@ rpm) 210 @ 3,800 335 @ 6,000 382 @ 6,000
Torque (lbs.-ft. @ rpm) 398 @ 1,600 - 2,400 339 @ 2,700 - 5,000 391 @ 2,800 - 4,800
EPA gas mileage (mpg, city/hwy) 17/23 13/18 13/17
Driveline AWD AWD AWD
Transmission 7-speed auto 7-speed auto 7-speed auto
Zero to 60 mph (sec.) 9.1 7.4 6.4*
Required fuel Diesel Premium gas Premium gas
*Mechanically identical 2008 GL550; 2009 GL550 acceleration figures were unavailable as of publication. All other figures come from automaker and EPA data for 2009 models.

Due in large part to supply-and-demand issues, diesel fuel was 68 cents more per gallon than premium unleaded — $3.19 versus $2.51, according to AAA's national averages as of Nov. 6. Unfortunately, that almost exactly cancels out the mileage advantage. To neutralize emissions, the Bluetec system uses a liquid solution that Mercedes calls AdBlue, a term far more marketable than the stuff's trade name, urea. The AdBlue reservoir needs to be refilled every 10,000 miles or so, something Mercedes bundles into routine dealer maintenance. I called a few dealerships to see if the procedure affects the cost of such maintenance; a dealership near New York said AdBlue costs extra but wouldn't say how much. A dealership in Los Angeles said it does not cost extra, while one in Chicago quoted me $32 per AdBlue refill. Be sure to ask your salesperson for an exact estimate and hold the dealership to his or her quote.