The Atlanta, Georgia-based company said each of the 50 hybrid electric vehicles, or HEV’s, will save some 44,000 gallons of fuel annually.

Apart from emitting 16,000 fewer tons of carbon dioxide annually, the 4,100 low-emission trucks are also 15 percent more fuel-efficient and will save UPS 1.5 million gallons per year, the company said.

UPS, the world's largest package delivery company, has a global fleet of around 91,000 vehicles.

The company would not disclose how much the new trucks will cost, but spokesman Norman Black said the HEV’s generally cost twice as much as ordinary trucks.

"But the idea is to recoup that investment through fuel savings," he said.

The HEV’s will reduce UPS' emissions by 457 metric tons of carbon dioxide per year.

A hybrid vehicle replaces a conventional engine with a small fuel-efficient diesel engine that acts as a generator providing energy for batteries and drive motors that power the vehicle.

The vehicles also contain regenerative braking, by which the energy generated from slowing or stopping the truck is captured and stored in the vehicle's battery.