THAT'S A HYBRID?” THE PARKING ATTENDANT had a tone of awe in his voice when I picked up the new 2005 GMC Sierra for a test drive in Boston. The Sierra and its twin Chevy Silverado are the first full-size hybrid pickups. GM hopes builders will be enthusiastic—and willing to spend an extra $2,500 for 10 percent to 15 percent fuel savings and for an on-demand AC generator.

To save gas, an AC motor starts the truck's gas engine, helps it accelerate and shift gears, and runs the power steering and brakes. It also shuts down the engine when it's not needed. This takes some getting used to: As I stopped in traffic, I was sure the engine had stalled. But when I took my foot off the brake it quietly came to an idle, then moved forward smoothly, with no power loss.

The electric motor also powers ground, fault-protected AC outlets in the crew cab and truck bed. To test their appeal, we loaned the truck to builder Steve Hughes with Barr and Barr Construction in Framingham, Mass. “I had six carpenters working off of it,” he says. “They were running a chop box, a circular saw, reciprocating saw, and a hammer drill.” The power was comparable to an AC wall outlet. “And the idling engine was very quiet.”

Does the truck justify its cost? “It would be good to have it as one of our company trucks,” says Hughes. “It would be handy for small jobs or for before you have permanent power.”

He has time to think it over. GM will deliver only 1,500 hybrids this fall to dealerships in limited states: California, Washington, Oregon, Nevada, Alaska, and Florida. Look for wider distribution next year.

Charles Wardell is a freelance writer based in Vineyard Haven, Mass.