Want a surefire tip to save gas? Buy a diesel. And now is a great time -- a wave of compression-ignition SUVs is washing ashore. One of the nicest is Mercedes' GL320 CDI.
Diesels have a longer expansion ratio (flip side of the engine's compression ratio), making each piston stroke like a longer lever. Plus, they don't exhaust unburned fuel out the tailpipe, they return it to the tank instead, which makes diesels more efficient than gas engines. Plus, diesel fuel packs 10 percent more energy per gallon than gas, though most of this advantage is unfortunately lost to diesel's higher price.
Running just six cylinders with only 3.0 liters of displacement, the Mercedes may appear too puny to tow a 24' runabout such as my Galaxy. But this engine delivers a strapping 398 lb-ft of torque, available at a mere 1600 rpm, plus its seven-speed automatic transmission multiplies that output, making for a formidable towing rig. Acceleration and passing ability are impressive. But what really rang my bell was the surprisingly good 17 mpg I logged during my test. That's the highest efficiency I've recorded for this column in years.
Another unsolicited vote in favor of modern diesels came from my wife, who exclaimed, "That's a diesel?" Yes, dear, those smelly days of ratty, sluggish diesels are long gone.
Wrapped around this impressive powertrain is the biggest Mercedes-Benz on the road, roughly the same size as a Chevy Tahoe. It seats seven in three rows and provides easy entry through long back doors. The power-operated rear gate and third-row seats are handy. Fold the two back rows flat and there's enough space for two people to sleep side by side.
Interior trim is first rate with real wood, chrome and brushed-metal accents, and fastidiously stitched leather goods. Seats are supremely comfortable and the Harman-Kardon 11-speaker sound system is superb for cranking up the tunes. Even a novice could master the backup camera, which is perfect for spotting the tow ball under the trailer hitch on the first stab. With patience and time, it's even possible to master the 37 push-buttons clustering the navigation screen.