Power, efficiency merge in BMW’s diesel X5

Looking at the fuel-economy numbers of 19 miles per gallon city and 26 mpg highway, you’d say “Big whoop — get that on Honda Accord or Chevy Malibu sedans.”

But here’s the gee-whiz part: The BMW X5 xDrive 35d is no midsize sedan. It’s a 5,200-pound full-size sport-utility vehicle. The X5 carries seven passengers and has the power to smack down any V-8 on the road — yet it gets midsize-car fuel economy.

This X5’s clean little secret is its high-tech turbocharged inline six-cylinder diesel engine, a power plant almost 20 percent smaller than the V-6 in an Accord or Camry. But thanks to the almighty shove from its sequentially turbocharged I-6, the BMW X5 performs better than a V-8.

Diesels generate gobs of torque, and torque — not horsepower — is the force that accelerates you away from a stoplight. That’s why every long-haul truck has a diesel.

But the X5 xDrive 35d’s diesel is no truck engine; it idles just a little more audibly than a gasoline engine, but the X5’s engine bay and cabin are so artfully encapsulated, you never hear a thing from inside.

The other reason big rigs use diesel is because it’s 25 percent to 30 percent more efficient than gasoline engines. That’s how the X5 xDrive 35d (aka “the X5”) manages the giddyup of a V-8 with the economy of a six-cylinder. Torque this momentous is an incredible empowerment: Tip into the throttle at 80 and the diesel-motivated X5 leaps forward like it’s been struck by Thor’s hammer. The diesel six-cylinder generates 425 foot-pounds of torque, compared with the X5’s 4.8-liter gasoline V-8 that produces 350 footpounds of torque.

Better still, the window-sticker EPA numbers are conservative. The X5’s economy readout typically showed 30 mpg or more as it cruised like a locomotive at 80 mph. (The X5 V-8 gets 14 and 19 mpg, respectively).

Inside the X5, you’ll conjure images of a private jet. Four of us soared along the interstate for 90 minutes with barely a whisper of the air moving past the windows. The diesel labors so effortlessly that it felt as though the X5 weren’t touching the ground. We were swallowed by the lush-leathered comfort seats ($1,200) and the surprisingly supple suspension. Thankfully, this X5 rides on sensibly sized 18-inch tires, absorbing all but the most horrid road warts.

You might remember car diesels as smelly, noxious things, but your memory wouldn’t be serving you well. Today’s diesel is packed with advanced new technology that simultaneously quiets, cleans and produces all that torque.

One problem — until now — has been that diesels produce an excess of oxides of nitrogen (NOx) emissions, the stuff that helps causesmog. The most important new addition is a system that injects urea fluid into the exhaust stream, enabling the X5’s special catalysts to scrub the NOx so clean that even California regulators can’t complain, thus enabling BMW to sell the X5 in all 50 states.

This is such a magnificent transportation object that I hate to bring up that I didn’t like the turbo lag. Because diesels use turbochargers, you get turbo “lag,” a brief moment it takes the turbocharger to begin its job. In the X5, you prepare to pull into traffic, squeeze the right pedal and - for an uncomfortable half-second - nothing happens. The 2 1/2 tons of luxury SUV just sit there.

Then, barely before you realize all is not as you’d like it to be, the turbos awaken, and the X5 leaps like an attacking Doberman. Still, I think BMW engineers could calibrate the X5’s throttle to better deal with the turbo-lag phenomenon.

The 2009 BMW diesel X5 starts at $51,200 ($65,000-plus on our tester’s bottom line). But I’ll say this: Diesel is the real deal, particularly for large, heavy vehicles like the X5. The X5 already was a fine SUV, but that formidable new diesel engine is what makes the xDrive 35d worth its price.