Traditionally, luxury SUVs and fuel economy have gone together about as well as Republicans and gay marriage. But recently, buyers of these behemoths have begun adding smaller carbon footprints, more miles per gallon, and fewer stops at the gas station to their already lengthy list of demands. The vehicles here are luxurious, but they are affordable enough to land in homes where practicality is still a concern.

And so luxury automakers -- who also have seen the writing on the wall on Capitol Hill, which suggests that they'd better get their fuel economies up or risk forfeiting a chunk of their high profit margins as fines -- have responded with measures to increase fuel efficiency in the form of high-tech injection systems, zillion-speed transmissions, diesel engines, and in some cases, all those things.

We set out to find which among the lux-u-vees had the best fuel economy of the bunch as tested by the EPA along its redefined guidelines, said to reflect real-world driving conditions better than its previous method. We defined "luxury SUV" the same way we do for our annual 5Best Trucks competition: SUVs with a base price over $35,000 and at least one model with a base price over $45,000. For example, the Cadillac SRX, which has a base-price range from $37,790 for a rear-wheel-drive V-6 model to $46,665 when equipped with all-wheel drive and a V-8, is eligible. The Lexus RX line, which starts at $38,165 for a front-wheel-drive V-6 and rises to $43,345 for the all-wheel-drive RX400h hybrid, is not.

So if you're as curious as we were, or if you happen to be in the market for a vehicle that spoils you with creature comforts without forcing you to fill up every two days, read on.

Mercedes-Benz ML320 CDI, GL320 CDI, & R320 CDI

Sweeping the podium is a trio of diesel Mercedes trucklets. M-B's diesels already have a strong following in Europe, and the company is starting to leverage that clout in the U.S. with three SUVs powered by its stellar turbo-diesel 3.0-liter V-6, which produces a respectable 215 horsepower and a trailer-tugging 398 pound-feet of torque. All are mated to a seven-speed automatic transmission and all-wheel drive. No matter which body it's dropped into, the diesel mill delivers buckets of smooth passing power and silent highway cruising, even if it's accompanied by a rather unsexy moan at full throttle. And all three, interestingly, achieve the same 18/24 fuel-economy figures from the EPA -- hands down the best in the field.

Astute readers may wonder how the biggest, heaviest, and truckiest of the three, the three-row GL320 CDI, can score the same fuel economy as the lighter, smaller ML320 CDI and the slippery, minivan-like R320 CDI. We wondered the same thing. But as it turns out, there is a subtle difference: Whereas they all have the same city and highway ratings, the GL is marked with a combined 20-mpg rating versus the others' 21 mpg combined.

Best of all, unlike many other vehicles with optional diesel engines where the diesel variants cost a boatload more money to buy than their gasoline counterparts, the diesel versions of the R-class and M-class cost only a grand more than if they were equipped with the 268-hp, 3.5-liter gas-powered V-6, and the GL diesel actually costs $2500 less than its base gas version, a 335-hp, 4.7-liter V-8.

For now, these diesels are legal in only 45 states, but by fall of 2008, the BlueTec AdBlue treatment system should be in place, enabling all three to pass emissions tests from Maine to California.

Cadillac SRX & Infiniti FX35

Tying for the most-fuel-efficient gasoline-powered vehicles in the group are two sports-sedan-based crossovers from opposite sides of the Pacific. Both offer V-8s and all-wheel drive but come standard with more-fuel-friendly V-6s and rear-wheel drive.

The Cadillac SRX's wagon-like boxiness may polarize audiences, but it's crisp, modern, and relatively fresh-looking even four years after launch. The chunky exterior encompasses a spacious three-row, seven-passenger interior that went through an effective redesign and material upgrade for 2007.

When we've sampled the SRX in the past, we've found its handling to be delightful, although the 3.6-liter V-6 is merely adequate in its delivery of its 255 horsepower and 254 pound-feet of torque. However, liveliness is an entirely different measure of performance than the accomplishment of 15 city and 22 highway mpg, which we speculate will be of equal importance to many of today's luxury-ute shoppers. Also, all-wheel-drive SRX V-6s only penalize their drivers by 1 mpg in the city, achieving a rating of 14 and 22.

Infiniti's FX hasn't changed much in the five years it's been with us, and during that whole time, it has offered some of the best fuel economy in its class. Funny we didn't notice. We were too busy charging up and down our favorite roads enjoying the beautiful performance of the FX's prolific G35-based chassis, as well as its 280-hp V-6. The "bionic cheetah" styling still looks good, and we wonder what more could be done for fuel economy (or performance, for that matter) if the five-speed automatic were to have one more gear.

TIE BREAKER: Besides offering more seating and a huge optional sunroof, the SRX achieves its fuel economy on regular unleaded; the Infiniti on premium.

BMW X5 3.0si

The X5 is celebrated as one of the world's best-performing luxury SUVs, but one of the most fuel-efficient? We were surprised, too. Credit the technology in the X5's 3.0-liter inline-six -- the precise fuel metering allowed by variable valve lift, as well as variable valve timing on both intake and exhaust cams. The tall top gear of the six-speed automatic also helps keep revs down while slipping through the atmosphere at highway speeds.

While the powertrain is busy eking the most out of every drop of gas, the driver enjoys uncanny road feel, precise and natural-feeling steering (at least when not equipped with the optional active steering), and one of the best of BMW's modern interiors. We suggest not ordering the spurious third-row seat, which reduces cargo space while adding weight and, therefore, sapping fuel economy. Also, although one of the most efficient SUVs on the road, the X5 3.0si is hardly among the cheapest, with prices charging toward $60,000 without much difficulty once a few options boxes are checked. Still, it's one of the best-driving SUVs at any price, and the fact that it's relatively efficient is a sweet bonus.

Mercedes-Benz ML350, Mercedes-Benz R350,
& Acura MDX

Tying a few mpg back are two more Mercedes-Benz utes, the ML350 and the R350, as well as the Acura MDX.

Redesigned for 2007, the Acura MDX went from dowdy suburbanite to Tomorrowland style statement at the flick of a light switch. Honda and Acura are known for wringing optimal performance out of every cube of displacement, and true to form, the MDX's 3.7-liter V-6 produces an impressive 300 horsepower and 275 pound-feet of torque while achieving laudable 15 mpg city and 20 highway. The standard Super Handling All-Wheel Drive makes the MDX a superstar on tight roads, and its spacious, high-zoot seven-passenger interior makes it a superstar on carpool day. No wonder it took top honors in our recent comparison test of $50,000 SUVs.

The two other vehicles here are a pair of Benzes closely related to the diesels at the top of this list: the gas-powered R350 and ML350. Between the two, the ML350, with its two rows of seats, high-up driving position, and all-wheel drive, is much more the traditional luxury SUV and is indeed the successor to the first import-brand luxury SUV, the original 1998 ML320. The R350, on the other hand, with three spacious rows of seats, is the ultimate minivan, or the ultimate on-road executive transport device, given that its third row can actually fit adults. Both, however, share the same sinewy 3.5-liter V-6, which produces 268 lively horsepower and 258 pound-feet of torque, as well as spot-on steering, car-like handling, and obviously, decent fuel economy.

Audi Q7 3.6, Porsche Cayenne, Saab 9-7X,
Volvo XC90 3.2, & VW Touareg

Okay, when consumption figures sink to 14 mpg city and 20 highway, it's hard to wax endlessly about fuel economy. Then again, considering the weight, power, and performance capability of some of these five vehicles, luxury SUVs all, the fact that their fuel economy isn't worse is noteworthy in itself.

Three of these vehicles, the Q7, the Cayenne, and the Touareg, share a platform co-developed by Porsche and Volkswagen. All three offer more powerful engines for people concerned less about fuel economy and more about haste. V-8s are available in all, with the Cayenne offering a choice of naturally aspirated or turbocharged, for up to 500 gasoline-unfriendly horsepower. Even with the base V-6s, though, the trio offers excellent driving dynamics for SUVs.

Saab's 9-7x is a barely luxurious version of a seven-year-old platform that has worn badges from nearly every brand in the General Motors fold, including Buick, Chevrolet, Isuzu, GMC, and the late Oldsmobile. Its 4.2-liter inline-six impressed us with an excellent blend of power and efficiency seven years ago, and although the power is still decent, this engine has been surpassed by plenty of competitors in both regards.

The Volvo XC90 is an otherwise decent vehicle with a pleasant, peaceful interior and a sedate highway ride, but with the base inline-six, it is slow. With any options, the price swells quickly. If it were our money, we'd take a hit of 1 mpg in both city and highway ratings and spring for the 311-hp V-8.

This is where the field is now. But it won't be this way for long, as more and more luxury automakers have announced that they will jump onboard the hybrid bandwagon with Lexus. Soon enough, we will have a Porsche Cayenne hybrid, a BMW X6 hybrid, and even a Cadillac Escalade hybrid, plus an Audi Q7 with a miserly 3.0-liter diesel V-6. Some or all of those could achieve fuel economy to compete with the mighty diesels from Mercedes and thus qualify as some of the most-fuel-efficient luxury SUVs in the world.


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