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.