Package delivery giant UPS has bought 48 heavy tractor trucks equipped to run on liquefied natural gas.

UPS said it expects the 450-horsepower tractors will produce 25 percent fewer greenhouse gas emissions and displace 95 percent of the fuel used by conventional diesel vehicles.

While an LNG-operated truck is more expensive -- about twice as much as conventional diesel -- its operating costs are about 50 percent less, UPS says. LNG fuel is less than half the price of diesel per gallon.

"This is an important step not only from an environmental standpoint but from the viewpoint of U.S. energy security," said Mike Britt, UPS's director of vehicle engineering in a news release.

"Liquefied natural gas is a cheaper, cleaner-burning fuel that is better for the environment and more sustainable than conventional diesel. And it's also a fuel that's in abundant supply inside the United States; it doesn't have to be imported."

Because LNG is very dense, it is particularly suited to large trucks that need to travel a long distance before refueling, Britt said.

Using natural gas as the primary fuel, the LNG technology for the UPS tractors includes a small amount of diesel as a combustible element to ignite the natural gas. A computer will also add diesel when it senses the engine requires it for lubrication. Still, diesel use will be reduced by about 95 percent.

Along with UPS's 11 existing LNG-powered tractors, the 48 new tractors will pull trailers on an Ontario, Calif.-Las Vegas transit route.

UPS, which has headquarters in Atlanta, has contracted with Clean Air Fuels Corp. to build a publicly accessible LNG fueling station in Las Vegas for the fleet. Annual fuel requirements for the new tractors are expected to exceed 1.2 million gallons.

The new fleet "is part of continuing efforts by UPS to reduce its emissions from the use of fuels like gasoline and diesel, and to lower its carbon footprint," said James Harger, chief marketing officer of Clean Energy.

UPS says it is the only private delivery company using this LNG technology in its fleet.

The company operates 1,914 alternative fuel vehicles, which together have traveled more than 185 million miles since 2000 in the United States, Canada, France, Germany, Brazil, the United Kingdom and Hong Kong.

The International Association of Natural Gas Vehicles estimates that there will be more than 50 million natural gas vehicles worldwide within the next 10 years.