Now that hybrids have a firm green foothold on the market for cars and light duty trucks, they are moving into medium- and heavy-duty trucks.

And while they are still expensive, they are getting more economical by the day as diesel prices continue at or near record levels.

Just about every manufacturer of medium- to heavy-duty trucks, including majors like Freightliner TRUCK, Kenworth Truck and Peterbilt TRUCK, are developing or producing larger hybrid diesel-electric trucks, Truck Manufacturers Association in Washington, D.C. Several companies began producing them this year.

Hybrid diesel-electric trucks are expensive - typical for new technology - but manufacturers say the vehicles can save 30 to 60 percent on fuel costs, depending on how they are used. They also reduce diesel emissions and should help quiet truck and bus noises. Dealers expect the costs to come down in time.

“Think everybody sees the benefit of doing this for certain vocations,” “Some vocations will benefit more than others, but all can benefit by using hybrids to some extent.”

Hybrids have become popular when it comes to cars, pickup trucks and SUVs. They made up about 2 percent of the total U.S. vehicle market last year, but U.S. registrations of new hybrid vehicles rose 38 percent last year from 2006.

Clarke said the diesel-electric technology is ideal for delivery and garbage trucks and buses and other stop-and-go urban uses because the more the vehicles stop, the more their batteries get charged. The technology has been around for some time, but mostly for mining and military uses.

Like cars, hybrid trucks rely more on electric power at low speeds.

“One application folks are looking at a lot is utility bucket trucks, such as those that operate on power lines,” “Sometimes, they just have to operate the lift, so the engine is running. If all that can be operated by batteries, the vehicles don’t have to sit and idle.”

Dean Swick is sales manager for medium-duty vehicles in Kenworth’s Western region. diesel-electric bucket truck could reduce its idling time by 85 percent and save up to 60 percent in fuel.

So far, the diesel-electric hybrids cost up to 60 percent more than a comparable diesel-only truck.