Until very recently, the prospect of getting 21 mpg in a full-size, truck-based SUV would seem rather fanciful. Normally, you'd be hard-pressed to get 14 mpg in one of these over-5,000-pound beasts. Yet getting 50 percent better fuel economy without giving up a powerful V8, eight-passenger capacity and the ability to tow 6,000 pounds is exactly what the 2010 GMC Yukon Hybrid is all about.

A couple of years ago General Motors accomplished this with the debut of hybrid versions of its popular Chevy Tahoe and GMC Yukon twins. It got some help with the "two-mode" powertrain that was developed via a joint venture involving BMW, GM and the former DaimlerChrysler. Starting with GM's 6.0-liter V8 (with cylinder deactivation that can shut down four cylinders under light-load conditions), the system adds a pair of 60-kilowatt motors (packaged within the transmission) for electric motivation. The transmission is rather complex, as it is essentially like having two transmissions inside one -- a continuously variable drive unit for light load conditions and a standard four-speed fixed-gear type for high-load conditions.

To optimize its fuel efficiency, the Yukon Hybrid can move solely under electric power, under low-load conditions to speeds up to 25 mph. This is how its city fuel economy rating (21 mpg) manages to virtually match its highway estimate (22 mpg). To minimize the weight gain associated with all that hybrid hardware, GM utilized aluminum for several body panels and even slimmed down the seats. Strangely, the back-breaking-to-remove third-row seats didn't take part in the diet.

Though the 50 percent improvement in city fuel economy over a standard Yukon is impressive, the Yukon Hybrid has its downsides. One is shown on the sticker, where the numbers range from the low- to the mid-$50,000s – that's about $8,000 more than a fully loaded Yukon SLT. And there are also the powertrain components to consider, which add weight and complexity. We'll let you decide if the environmental benefits are worth it, but how green can a 5,600-pound SUV ever really be?

So, unless the 2010 GMC Yukon Hybrid's significant towing capabilities are important to you, a full-size crossover like the Buick Enclave or GMC Acadia is likely a better choice. They provide more usable passenger space, are friendlier to drive, get close to the same fuel economy and are considerably cheaper when fully loaded with options. You might also take a look at smaller but similarly priced diesel-powered three-row crossovers like the Audi Q7 TDI and BMW X5 xDrive 35d.

The 2010 GMC Yukon Hybrid is a full-size SUV available in one trim level. Standard equipment includes 18-inch alloy wheels, rear park assist with rearview camera, tinted windows, power-folding heated side mirrors and tri-zone automatic climate control. A trip computer, Bluetooth connectivity, an auto-dimming rearview mirror, remote ignition, leather upholstery, power front seats and a removable 50/50-split third-row seat are also standard. In-car entertainment includes a navigation system, a hybrid system display and a nine-speaker Bose audio system (with a CD/MP3 player, a USB port and satellite radio with real-time traffic reporting). A sunroof and a rear-seat DVD entertainment system are the lone options.

The 2010 GMC Yukon Hybrid is available with rear-wheel or four-wheel drive. Both models utilize a 6.0-liter V8 engine coupled to a pair of 60-kilowatt electric motors located inside what GM calls an electrically variable transmission. Together, they produce 332 horsepower and 367 pound-feet of torque. The system can accelerate the Yukon up to speeds of approximately 25 mph using electricity only, while the V8's cylinder-deactivation system helps reduce fuel consumption at higher speeds. Regenerative braking replenishes the batteries by capturing energy normally lost when you come to a stop.

Fuel economy ratings stand at 21 mpg city/22 mpg highway and 22 mpg combined for rear-wheel-drive Yukon Hybrids, with 4WD versions earning identical numbers except combined, which is just 1 mpg less. Maximum towing capacity for a properly equipped 4WD model is 6,000 pounds.

Standard safety equipment includes full-length side curtain airbags, antilock disc brakes, traction control, OnStar and a rearview camera. In government crash testing, the Yukon Hybrid scored a perfect five stars in all frontal and side-impact tests.

It's not a stretch to say that driving the 2010 GMC Yukon Hybrid feels like being behind the wheel of a 5,600-pound Prius. There's the same eerie quiet when accelerating and braking, as the gas engine shuts off to let the electric motors do their thing. Although it's a tad strange, the result is a quiet cabin, while transitions between gas and electric modes, and eight- and four-cylinder mode, are either undetectable or easy to ignore.

Although the Hybrid is the most powerful Yukon available, it's also the heaviest, so don't expect particularly brisk acceleration. Also, the transmission isn't what we'd call responsive; there can be a notable delay when you ask for full power. Handling is about what you'd expect -- safe but ponderous. Most crossovers are notably more carlike from behind the wheel. The Yukon's cabin remains fairly quiet at speed, though, and the ride is relatively smooth for a truck-based SUV.

Aside from instrumentation, there's nothing to distinguish the Yukon Hybrid from a traditional Yukon. Unlike in past generations, that's a very good thing. The newest Yukon boasts attractive, high-quality materials and tight panel gaps while maintaining a simple control layout. Even the standard navigation system is easy to use.

The Yukon's standard third row enables it to accommodate up to eight passengers. The 50/50-split third-row seats don't fold flat into the floor, however; they must be removed manually to free up maximum cargo space, and each seat weighs more than 60 pounds. With the third-row seats out of the picture and the second-row seatbacks folded, cargo capacity expands to a whopping 109 cubic feet, making the Yukon the roomiest hybrid on the market.