CARY - Hybrid engines can double a typical vehicle’s mileage – and that’s how one Cary trash truck is pushing the four-mile-a-gallon mark.

The town is halfway through its assessment of one of the United States’ first waves of hybrid-engine garbage trucks. Cary’s model pressurizes hydraulic fluid as it brakes, then uncaps that pressure to provide forward motion.

The truck doesn’t look any different than a normal trash loader, and its gas mileage may sound abysmal, but it’s turning out to be a significant improvement. Most trash trucks get about two miles per gallon of gas.

“We’re interested to see if fuel efficiency would pay for the higher cost of the vehicle,” which is about 30 percent pricier than a normal truck, said public works director Scott Hecht. “Right now, six months in, we’re seeing about 100 percent fuel efficiency” improvement.

Garbage trucks have such poor gas mileage because they weigh about 60,000 pounds full, and because they’re constantly accelerating and braking.

“They’re starting and stopping a thousand times a day,” Hecht said. “They’re not even driving 30 mph, much less 50.”

The hybrid’s extra two miles to the gallon can really add up over a truck’s 10-year life span – and the real test will be if it can eventually save enough to counteract the extra $100,000 on the price tag.

Typical trash trucks cost about $285,000. The Triangle J Council of Governments paid for the price difference on Cary’s first truck as part of a pilot program, but the upgraded truck will have to cut mustard all by itself.

Most municipalities, including Cary, have begun to experiment with alternative engines for their fleets of heavy machinery, according to Kathy Boyer, the energy and environment program manager for the Triangle J Council of Governments.

“People are finding the business case now that petroleum prices are so high, but the technology itself has been available to some degree for probably 20 years,” Boyer said.

The hydraulic hybrid engines also have found new popularity thanks to the American Recovery and Reinvestment Act, also known as the federal stimulus, which gave the Triangle J $12 million to disburse for alternative-fuel transportation in North Carolina and South Carolina.

Besides potential savings on fuel, the grants also are an effort by governments to set the tone in discussions about energy efficiency and emissions reduction.

“If we’re expecting our commuters to be able to reduce their petroleum use, than our local governments have to as well,” Boyer said.

Cary will continue to assess its diesel-hydraulic truck for another six months before staff start making some final conclusions.

For now, Hecht said, the truck’s just one of the fleet.

“She’s out every day but Monday.”