CHARLOTTE, N.C. -- FedEx has added 92 hybrid-electric trucks to its delivery fleet, which are the first standard FedEx delivery trucks converted to hybrid-electric systems.

The addition of the 92 trucks increases the FedEx fleet of hybrid-electric vehicles by more than 50 percent, from 172 to 264.

The FedEx hybrid-electric fleet has logged more than four million miles of revenue service since being introduced in 2004, reducing fuel use by 150,000 gallons and carbon dioxide emissions by 1,521 metric tons, which is equivalent to removing 279 cars from the road annually.

The hybrid conversions were produced in Charlotte, N.C., during the past six months. The converted hybrids were developed with Freightliner Custom Chassis Corporation and Eaton Corporation, which provided the hybrid-electric systems.

In converting the standard delivery vehicles, the power-train equipment, including the engine, transmission, fuel tank and drive shaft, were replaced with a 2007 Cummins ISB 200 hp engine and Eaton hybrid-electric system.

The vehicles feature a diesel engine coupled with an electric motor/generator and lithium-ion batteries. These batteries capture and store energy during the regenerative braking phase of vehicle operation, eliminating the need to plug into an electrical source.

The sophisticated hybrid controller selects the most efficient mode of operation -- diesel or electric -- depending upon current operating conditions and driver demand.

The standard FedEx trucks used in the retrofit program were 2000 or 2001 models with 300,000 to 500,000 miles driven.