A single-mode hybrid system is sufficient for small high-mileage cars but, when you're dealing with husky, fuel-slurping trucks, you need more. What we now know as GM's two-mode hybrid system was originally developed for urban buses that began service in 2003. As the technology trickled down to Chevrolet and GMC SUVs, pickups, and (future) crossovers, GM resisted the urge to coin a new name even though the actual number of driving modes increased as the hardware evolved.

Matching last year's Chevy Tahoe and GMC Yukon applications, two-mode applied to the GMC Sierra pickup consists of a 6.0-liter V-8 engine teamed with two 80-horsepower AC electric motors, three planetary gear sets, and four multi-plate clutches. Everything but the engine and a 300-pound, 300-volt nickel metal hydride battery pack is stashed inside an aluminum case that looks like an enlarged automatic transmission. The aluminum block and head Vortec V-8 operates with late intake valve closing (called an Atkinson cycle) and Active Fuel Management (GM's name for cylinder shut-down) to maximize mileage. The engine's peak output is 332 hp@ 5100 rpm and 367 lb-ft of torque at 4100, a slight reduction from the standard Silverado's 6.0-liter output. Since both electric motors pitch in when needed, the total hybrid-powertrain output and efficiency are both higher than what's delivered by a conventional powertrain.

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After all systems are warmed up, the Sierra's engine shuts down at stop lights and during low-level acceleration to 30 mph. During light-load cruising, four of the eight cylinders take a break. In normal driving, the gas and electric mix is seamless with the tach needle rising in synch with the speedometer as the drive system shifts between two continuously-variable ranges. Except for an occasional hint of electric-motor whine, there's little audible evidence of this hybrid system in action. It's notably quieter overall than any conventional powertrain.

With a trailer hitched and the throttle floored, the system climbs through four fixed ratios with a slight hesitation noticeable at 38 and 73 mph. There are instances when the tach needle hovers in a narrow rpm range while the speedometer continues climbing. Uninhibited by a trailer or law enforcement, this truck accelerates to sixty mph in 9.7 seconds and through the quarter-mile in 17.3 seconds at 85 mph on its way to a governed top speed of 99 mph.

Mileage benefits are significant. The 4wd editions scored 20 mpg in EPA city and highway tests or about 5 mpg better than a conventional full-sized V-8-powered pickup. The 2wd edition carries an even better 21 city, 22 highway mileage sticker.

Two-mode adds about $3500 to the price of a Sierra pickup and is offered only with a crew cab and a 6-foot bed (which has an actual floor length of 69.3 inches.) Why just a crew cab? Because the battery pack is bulky and requires fastidious temperature control so it can't readily be stashed under the bed floor. Also, it would consume too much interior space inside regular and extended cabs. While the cushion portion of the rear seat hinges up, the space available for cargo in the back of the crew cab is diminished by the battery pack.

Chevrolet offers the same basic package on Silverado crew cab pickups which begs the question, what's the real difference between a Sierra and a Silverado? Mechanically, they are identical. Interior decor is also common. The distinction is all in exterior design. Each of these twins has its own front fenders, hood, grille, and front fascia. The shape of the rear fender surfaces is also unique to each brand. For the privilege of shopping at a GMC dealer versus a Chevy showroom, you will spend an extra $370.

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Two trim levels are available. The base 2wd Sierra two-mode costs $39,365 including destination. Adding 4wd boosts the sticker by $3150. (StabiliTrak electronic stability control and a locking rear differential are standard on all Sierras.) The $6135 premium trim package adds leather trim, power front bucket seat adjusters, a center console, a Bose sound system with navigation, a hard bed cover, and several minor convenience items. The only optional equipment is a $995 moonroof with premium trim and power seat adjusters with base trim.

To study real world towing capabilities, I hauled a 23-foot SeaRay boat on a tandem-axle trailer on a 200-mile journey down Interstate 35 south of San Antonio, Texas. That payload weighed 5400 pounds, well within the 5900 pound tow rating GM assigned to a 4wd Sierra (lighter 2wd editions are rated for 6100 pounds). Hustling the rig to 60 mph from rest required only 18 seconds of smooth acceleration. A 30-60-mph passing maneuver took 13.6 seconds versus 7.3 seconds without the trailer attached.

While cruising on level ground, the engine stayed in 8-cylinder mode and there was no hint of strain holding 70-mph on grades. On downgrades, the driver information display reported an occasional shift to the V-4 running mode. There was no extra vibration or change in exhaust note during or after that transition.

I recorded 10 mpg while towing, about the same I've experienced towing similar sized boats with large conventional-powertrain pickup trucks. GM's Vehicle Line Director for full-size pickups Mike Tulumello acknowledges that two-mode offers minimal efficiency benefit on the highway and practically no mileage benefit while towing over standard powertrains.

The situation changes for the better in urban driving. In that circumstance, braking regeneration and electric propulsion combine to significantly curb fuel consumption. During a 30-mile tour of practically every San Antonio neighborhood, I achieved an amazing 25 mpg. Without causing any significant hindrance to traffic, the Sierra's electric motors handled propulsion duties most of the time with the engine starting only occasionally to power one of the electric motors operating in generator mode. After long runs using the electric motors, the battery is depleted so the engine must run to restore its state of charge.

As long as regular gasoline continues selling for less than $2 per gallon, two-mode - and for that matter, every other hybrid--is a tough sell, despite the $2200 energy credit that can offset federal tax liabilities currently offered on the 2009 GMC Sierra Hybrid. But this leap-ahead technology will soon enjoy its day. When gas prices again surge past $3 per gallon on their way to $4, investing a few thousand dollars to gain five or more mpg will seem like a genuinely shrewd move.

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