America's environmental chickens have come home to roost. For nearly two decades, we've blithely bounded from sea to shining sea in ever porkier sport-utility vehicles fueled by cheap gasoline. Now, many of us face the prospect of personal bankruptcy, not to mention social ostracization, if we continue to indulge in the privilege of carting home our groceries in three-ton trucks. But old habits are hard to break. Isn't there some way we can have our proverbial cake - lower emissions, less consumption of fossil fuels, Al Gore's approval - and eat it, too, by still driving big SUVs that can haul our families and tow our toys?

Well, Tipper, perhaps there is.

General Motors has spearheaded a new Two-Mode Hybrid system in association with Chrysler, Mercedes-Benz, and BMW. The system has been described in detail in these pages in recent months, and in our February issue, we gave GM our 2008 Technology of the Year award in recognition of its work on the system. (The Two-Mode Hybrid system, or TMH, recently debuted in the Chevrolet Tahoe and the GMC Yukon. It will also appear in GM's full-size trucks this fall and eventually in cars.) But what remained was to put TMH to the test in the real world, by driving two GMC Yukons - a conventional 2008 model with a 5.3-liter V-8 and a 2008 Yukon Hybrid, both with four-wheel drive - back-to-back over several days and hundreds of miles. We also brought along a 2008 Mercedes-Benz GL320 CDI, which is currently our favorite large diesel-powered SUV. (We know, we know, it's not like there are very many large diesel-powered SUVs from which to choose.) After all, in every discussion we've had over the past couple of years about alternative-fuel vehicles and the rising price of gasoline, diesel has been like a little monkey sitting on our shoulders, tapping its fingers and saying, "Hey, what about me?"

Let's cut right to the chase: as you can see in the chart below, the Yukon Hybrid crested the all-important 20-mpg mark on the freeway, achieving an average of 21 mpg versus the conventional Yukon's 16 mpg. But the diesel Mercedes did even better, averaging 27 mpg on the highway and, even more impressive, 23 mpg around town. The Benz's as-tested price, however, was much higher than the comparably equipped Yukon models; diesel fuel on average costs more than gasoline; and the diesel Benz is not sold in eight states because it doesn't meet their emissions standards. A fifty-state clean diesel - using urea-injection technology - is on the way for the Bluetec-badged Mercedes ML-, GL-, and R-classes this fall, however.

Jumping among the driver's seats of these three vehicles, as a number of magazine staffers did during our testing protocol, proved to be painless. With its ample reserves of torque, the diesel Mercedes is seamless to drive, and the 5.3-liter Yukon SLT has the easygoing nature of all GM's full-size SUVs and pickup trucks. What impressed us most about the Yukon Hybrid, though, is that it's actually better and more pleasant to drive than the conventional version, with stronger, more linear acceleration, even when towing. This is in stark contrast to most hybrid cars, such as the Toyota Camry Hybrid, whose driving experiences are far less satisfying than their conventional-powertrain counterparts.

Sure, when you let up on the Yukon Hybrid's gas pedal, there's that typical bit of drivetrain whine that you get in many hybrid vehicles, such that at first you think there's a siren wailing in the distance, but overall this is a very well-integrated drivetrain. The Yukon Hybrid's electric power steering, which runs on its own 42-volt system, is especially satisfying for such a big vehicle, thanks partly to a mass-damped steering wheel that the conventional Yukon does without. The brakes, controlled and applied by a mixture of electronics and hydraulics, are tuned for a very natural feel.

The TMH system adds about 400 pounds to the Yukon's weight, which GM partly offsets by eliminating the starter, alternator, torque converter, roof rack, spare tire, and jack and by using aluminum for the hood and liftgate. The Hybrid's thin-profile front seats are also lighter. As you can see in the photo, the Yukon Hybrid gets its own aerodynamic treatment, too, including a new grille, fascia, hood, and running boards.

There are compromises, of course. Our Yukon Hybrid cost about $4000 more than the conventional vehicle, and the second-row seats, which cover the battery pack, will not fold flat. When you flip those second-row seats forward, there is no mechanism to keep them in place, so they rock back and forth under sudden braking. (GM is aware of that problem and is seeking a fix.) Neither Yukon's third-row seats are as comfortable or as accommodating as those in the Mercedes. And while the Yukon Hybrid's towing performance was exemplary while moving forward, reversing the vehicle up a steep driveway proved to be a challenge even with nothing attached to it: GM's two-mode transmission doesn't include a reverse gear. All rearward propulsion is accomplished via one of the system's electric motors, so backing a loaded trailer up a grade could be problematic.

On the face of it, 14 mpg in the city and 21 mpg on the highway are probably not figures that will have the editors of the Green Car Journal dancing in the streets. But everything is relative: the Yukon Hybrid's fuel economy gains over its conventional sibling are tangible and considerable, and the driving experience has been enhanced, not lessened, by the addition of the Two-Mode Hybrid system. What really surprised us, though, was that the Mercedes diesel exceeded even its EPA fuel economy ratings in both the city and highway cycles, and in the process achieved mileage that would do an ordinary family sedan proud. That seems like something even Al Gore could get excited about.

The Specs	GMC Yukon SLT	GMC Yukon Hybrid	M-B GL320 CDI
Price (Base/as tested)	$40,465/$51,965	$53,755/$56,045	$53,775/$69,375
POWERTRAIN
Engine	OHV 16-valve V-8	OHV 16-valve V-8	Turbo-diesel DOHC 24-valve V-6
Displacement	5.3 liters (325 cu in)	6.0 liters (364 cu in)	3.0 liters (182 cu in)
Horsepower	320 hp @ 5200 rpm	332 hp @ 5100 rpm	215 hp @ 3800 rpm
Torque	340 lb-ft @ 4200 rpm	367 lb-ft @ 4100 rpm	398 lb-ft @ 1400 rpm
MEASUREMENTS
Weight	5840 lb	5900 lb	5720 lb
Tow Rating	8200 lb	6000 lb	7500 lb
FUEL ECONOMY
EPA Mileage	14/19 mpg city/hwy	20/20 mpg city/hwy	18/24 mpg city/hwy
Observed Mileage	11/16 mpg city/hwy	14/21 mpg city/hwy	23/27 mpg city/hwy

GMC has packed a lot into their Denali XT concept, giving us hints at what's next for the brand. The truck showcases a new take on car-based trucks, a possible future GMC styling direction, and a next-generation powertrain.

The Denali XT is a rear-wheel-drive sport utility truck (SUT) that rides on a unibody chassis - or what the Australians call a ute. Why bring up the folks from Oz when discussing a new GMC concept? It was Holden - GM's Australian arm - that designed the Denali XT for us Americans. They did a good job on the Camaro, why not let them make our trucks, too?

Australian or not, car-based trucks are nothing new for the U.S. El Caminos and Rancheros were sold for about two decades last century, and there's even Honda's Ridgeline - a truck loosely based on the Accord. What sets the Denali XT apart is its size; it's similar in length and wheelbase to a GMC Canyon crew cab, though wider and lower to the ground. The Denali XT only sends power to the rear wheels unlike the front- or all-wheel-drive Ridgeline.

With its flared fenders and high beltline the Denali XT has a decidedly muscular stance. In pictures, it looks bigger than it is, something we'll have a better idea of once we see it for real on the floor at the 2008 Chicago show. The grille gets a familiar treatment to the other Denali-trim versions of the Envoy, Yukon, and Sierra. The Denali XT's 23-inch wheels (known as Jordans to the street-credible in our office) are pushed to the corners and are attached via a height-adjustable air suspension.

As with most recent concepts, the Denali XT goes alternative in the powertrain department. It uses a version of GM's Two-Mode hybrid system, but for the first time couples it with a 4.9-liter small-block V-8 that's E85 ethanol-ready. The engine employs direct injection to get a power rating of 326 hp, as well as cylinder deactivation to save fuel. Like the other Two-Mode hybrids, the Denali XT can run electric-only at low speeds.

GMC has thrown all of its ideas at one concept and it seems to have worked. We expect much of what we see here to make it to production vehicles in the near future.

Click the link below for high-resolution Denali XT images.

Our 2008 Technology of the Year just got more work duty.

GMC has unveiled its Two-Mode 2009 Sierra Hybrid at the Chicago Auto Show.

As one of the first GM trucks equipped with the Two-Mode system (the hybrid technology also appears in 2008 GMC Yukon and Chevy Tahoe hybrids), GMC says the Sierra Hybrid achieves 40 percent greater city fuel economy and a 25 percent improvement in overall fuel economy when compared to the standard Sierra, which gets an EPA-rated 13 mpg city and 18 mpg highway. The hybrid version can reach speeds up to 30 mph on electric power and features additional fuel-saving technology, such as a 6.0-liter V-8 with electronic variable transmission (EVT) and GM's active fuel management.

Similar to its Two-Mode SUV cousins, the Sierra Hybrid has a 6100-pound towing capacity, is available in two- or four-wheel drive (StabiliTrak and a locking rear axle are standard) and benefits from subtle body enhancements - a slightly deeper front air dam and a tonneau cover to reduce air drag.

Inside the truck, drivers will notice a new instrument cluster and a battery pack located under the rear seat.

The Sierra Hybrid should be in showrooms by Fall 2008.

Click below for high-resolution Sierra Hybrid images.

Frankly, there's not a whole heck of a lot going on over at GMC this year. That's understandable, since its model lineup underwent such big changes in 2007 when all models, except for the ancient Envoy and people-hauler Savanna, were almost completely redesigned.

The GMC Yukon, like the Chevy Tahoe and Cadillac Escalade, will gain a hybrid model later in the year. On models with a front bench seat, there's a new storage cubby. Wait, come back! There are new chrome wheels too!

Likewise, the Yukon Denali and Yukon Denali XL, Sierra 1500 and Sierra 2500/3500 HD have very little changes for 2008. Most models get minor updates like standard XM satellite radio and new colors.

The Sierra Denali, which was not available last year, makes a comeback for 2008. It combines the Sierra's pickup body with the Yukon Denali's tarted-up interior, landing somewhere between the Chevy Avalanche and Cadillac Escalade EXT.

The impressive Acadia, which debuted last year, has some new colors and equipment updates. The second row of seats can fold flat into the floor, and you can also order a backup camera with the navigation system, which fixes one of the biggest complaints we have about our Four Seasons Acadia.

New Arrival
GMC Acadia
GM's big crossovers carry a big burden. We'll see how they handle it.The new, seven-seat crossover utility vehicles from General Motors are arguably even more important than the company's recent crop of full-size SUVs, which made their debut last year. After all, sales of traditional SUVs are declining, while the crossover segment is booming. The GMC Acadia, the Saturn Outlook, and the Buick Enclave must not only net people defecting from big SUVs, they also must pick up shoppers looking for a mid-size three-row utility vehicle, and, once the company drops its minivans, they'll need to capture those buyers as well. With so much riding on this trio, we had to get one in our fleet for a closer look. We went for the GMC, currently the top offering (the Buick won't be out until this summer). Our Acadia SL2 AWD is loaded with $7460 worth of options, including dual sunroofs, a DVD player, navigation, HID headlamps, nineteen-inch wheels, a towing package, and a head-up display. All those options are nice, but when the Acadia landed on our doorstep in the midst of a midwinter deep freeze, we were most grateful for two of its standard features: heated seats and remote start.

Specs
Price: $44,965Engine: 3.6L V-6Power: 275 hpTorque: 251 lb-ft

Notes
956 miles Senior editor Joe Lorio: "According to its trip computer, the Acadia averaged only 17.5 mpg on our four-hour, traffic-free sprint to Chicago. The V-6 often needed a downshift from the six-speed automatic to maintain between 80 and 85 mph. This is a big bus, but the upside is adult-rated seating in all three rows."1288 Lorio again: "I had a Lexus RX300 owner in the car and he was impressed with the interior. I am, too. It's tasteful, comfortable, and well-executed--but for one ill-fitting piece of trim above the right rear door. A more relaxed pace on the way home (just under 80 mph) produced much better mileage: 21.0 mpg."