A massive SUV that tops 20 mpg? Believe it. Using a next-generation hybrid propulsion system, the GMC Yukon, all 5,600 pounds and 202 inches, returns 21 mpg in city driving and 22 mph on the highway. Toyota Prius Hybrid owners who get 48 mpg city might scoff, but try stuffing seven or eight passengers in a Prius. Best of all, the Yukon feels and drives no differently from the gasoline-only version, yet gets 50 percent better mileage in town.

These aren't just your-mileage-may-vary guesstimates but the 2008 EPA calculations that bear some resemblance to reality. In a week of driving I averaged 19 to 23 mpg, depending on the type of trip. Gas-engine Yukon drivers filling up three times a month could be refueling every two weeks. Basically, you're getting all the goodness of the redesigned GMT9000 platform that two years ago took GM SUVs (Yukon, Chevy Tahoe, Cadillac Escalade) out of the dark ages of technology. And you even qualify for a hybrid energy-saver rebate from the government.

For a slideshow of the GMC Yukon Hybrid, click here.

How It Works in the Lab

While Toyota has been lapping up awards for its hybrids since 1999, GM for the past four years has been working quietly in a hybrid basic-research consortium in Michigan with DaimlerChrysler and BMW. During the period, GM also turned out some mediocre hybrids such as the last-generation Saturn Vue, mediocre more because of the platform than the simpler hybrid drivetrain GM employed. The gearbox technology GM brought home from the consortium and developed further for the Yukon and sibling Chevrolet Tahoe comprises four traditional fixed gears, two 60-kilowatt electric motors, electronically variable transmissions, and no reverse gear (instead, the electric motors spin backwards).

The Yukon draws power from a 6.0-liter V8 engine that can run on four or eight cylinders and the two electric motors that are driven from a 300-volt nickel metal hydride (NiMH) battery pack in the rear floor. Electric motors have maximum torque (turning power) at 0 rpm, making them useful for getting the Yukon moving from standstill. Conversely, they wimp out at high rpm, which the consortium worked around by driving them through reduction cars to keep motor speed low.

Offsetting 400 Extra Pounds

The hybrid Yukon carries around about 400 extra pounds, which GM offset using lighter materials elsewhere, ranging from aluminum in place of steel in some body panels, thin-profile seats, and tire pressure monitors instead of a spare tire. Also, the front was reshaped to be more aerodynamic. While the Yukon's drag coefficient of 0.34 is quite good, a vehicle 79 inches high and 77 inches wide gets in the way of a lot of air at highway speeds.

The upshot of the technology was this: The Yukon moved off smartly from standstill and had slightly more performance when accelerating. GM says it may be "one or two ticks faster [tenths of a second" faster to 60 mph than a non-hybrid Yukon. At stoplights the gas engine shut down but the accessories all kept running - lights, heated seats, the $1,295 DVD player option, air conditioning, navigation, crash sensors, power steering - drawing power from the 300 volt main powerpack that is converted down to 42 volts and 12 volts for some of the accessories. Yukon has no mechanical (belt driven) accessories.

With a gentle foot on the throttle, I could get to 25 mph and run for at least a half mile on the battery motor before the gas engine kicked in. This makes it a dual-mode, or strong, hybrid, meaning it can run on battery alone or gasoline alone, plus the two together. On the highway, the engine drops back to V4 mode on level roads, and the electric motors delay return to V8 power if you only need a bit more acceleration. All this is unnoticeable except for slight whirring noises at low speed and the ever-changing LCD display on the standard navigation system that show where the power comes from - gas engine, electric motor - whether the engine and brakes are regenerating the battery pack. There are no noticeable surges shifting modes.

Should You Buy?

With Yukon, Tahoe, and Cadillac Escalade (due for its hybrid the first half of 2008) General Motors makes the best full-size-plus SUVs and they're the way to go if you need to haul trailers up to 6,000 pounds (a bit less than the gas-only towing capacity) and a lot of people. Nothing else handles that combination. But if you want to carry seven passengers, no trailer, a Honda Odyssey or Toyota Sienna minivan is more space efficient and may have more cargo room.

The Yukon measures 202 inches long yet there's very little cargo space back of the third row and the seat cushion bottom rests almost flat on the floor atop the battery pack, providing far less comfort than a minivan. If you want reasonable cargo space, you need to step up to the non-hybrid Yukon XL model that measures 220 inches long. A review of a vehicle that large, painted white like my test Yukon, would better be written by Herman Melville.

Most equipment comes standard on the Yukon hybrid, leaving you only to choose the four-wheel-drive version for an extra $2,810 ($53,755) and a 20/20 city/highway mpg rating, sunroof ($995), and DVD player ($1,295). The essentially same Chevrolet Tahoe runs $455 less. If you're thinking big SUV, the hybrid version of the Yukon is well worth considering. Order it with the big "Hybrid" decals that came on my test car and your environmentally aware friends won't know whether to picket your driveway or ask for a test ride.

General Motors Corp.

Price: $50,945 to $53,225

EPA rating: 21 mpg city, 22 mpg highway

Pros: Great gas mileage (given its size). Good acceleration. Excellent underlying platform.

Cons: Costs several thousand dollars more than gasoline-only Yukon. Weight and frontal area keep fuel economy from mid- to high-20s. Meager third-row seating and storage (unrelated to hybrid issues).

Bottom Line: This next-generation hybrid drive helps GM's 3-ton SUV top 20 mpg city and highway. Except for occasional muted electrical sounds, you'd never know there's an exotic powerplant under the hood. The General is back.