A few years ago, the idea of a full-size traditional truck-based SUV averaging 21 mpg seemed about as likely as a Chevy Aveo blowing away a Corvette in the quarter-mile. Normally, you'd be hard-pressed to get 14 mpg in one of these over-5,000-pound beasts. Getting an amazing 50 percent better fuel economy without giving up a powerful V8, eight-passenger capacity and the ability to tow 6,000 pounds might qualify that rig as the eighth wonder of the world. That rig is the 2009 GMC Yukon Hybrid.

It was last year that General Motors achieved that feat, with the debut of hybrid versions of its popular Chevy Tahoe and GMC Yukon twins. Developed in concert by BMW and DaimlerChrysler and General Motors, the Yukon Hybrid's "two-mode" technology starts with a 6.0-liter V8 with cylinder-deactivation technology -- it essentially turns that V8 into a V4 when full power isn't needed. The V8 is coupled to a pair of 60-kilowatt motors packaged within the transmission for electric motivation.

The transmission -- GM dubs it an EVT, for "electronically variable transmission" -- is essentially like having two transmissions inside one -- continuously variable drive for light load conditions and a standard four-speed fixed-gear type for high-load conditions. Hence the term "two-mode." And like other full hybrids, the Yukon Hybrid can be propelled (up to around 25 mph) solely under electric power, which is why its city fuel economy (21 mpg) is virtually the same as its highway (22 mpg). GM did its best to keep the weight gain to a minimum by using aluminum for several body panels and slimming down the seats. (Though ironically, the oh-so-heavy removable third-row seats escaped the liposuction.)

The gains in fuel economy above a regular Yukon's are pretty impressive from a percentage standpoint -- the Hybrid posts an improvement in city mileage of about 50 percent. However, its mid-$50,000 price tag may make you think twice. Other than its sophisticated running gear, the Yukon Hybrid is essentially a fully loaded Yukon SLT, but costs about $8,000 more. Even factoring in federal tax credits, it could take up to a decade to recoup that price premium based on gas savings alone. We'll let you decide if the environmental benefits are worth the price, but how green can a 5,600-pound SUV ever really be?

So, unless the 2009 GMC Yukon Hybrid's significant towing capabilities are important to you, a full-size crossover like the Buick Enclave or GMC Acadia may be a better choice. They provide more usable passenger space, they're friendlier to drive, they get close to the same fuel economy and they're considerably cheaper when fully loaded with options. The new two-mode hybrid system is certainly impressive, but we're not entirely sure if the Yukon is the right vehicle for it.

The 2009 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, an auxiliary audio jack and satellite radio (with real-time traffic reporting).

A sunroof and a rear-seat DVD entertainment system are the lone options.

The 2009 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 coming to a stop.

Fuel economy is 21 mpg city/22 mpg highway for rear-wheel-drive Yukon Hybrids and 20/20 for those with 4WD. 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 2009 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.

The regenerative braking system produces a firm braking feel and, according to GM, actually stops the big SUV better than the regular Yukon's conventional brakes do. Although the Hybrid is the most powerful Yukon available, it's also the heaviest, so don't expect particularly brisk acceleration. Still, its abundance of low-end power -- aided by those torque-rich electric motors -- produces plenty of grunt for both around-town chores and trailer towing.

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.

With its standard third row, the Yukon can seat up to eight passengers. Unfortunately, that 50/50-split third row does not fold flat into the floor. Since the two seats weigh slightly less than a Geo, you'd better have Stone Cold Steve Austin on standby to help remove them. Once they're out, though, maximum cargo capacity is a whopping 109 cubic feet with the folding second row down -- much more than any other hybrid offers.