General Motors Corp. will double its fleet of hybrid-powered buses operating in the U.S. when the auto maker fills 1,732 orders from the Washington, Philadelphia and Minneapolis/St. Paul transit authorities.

General Motors shares a common goal with transit agencies in Washington D.C., Philadelphia and Minneapolis/St. Paul to improve fuel economy and reduce vehicle emissions with proven, durable hybrid technology,” says Mark LeNeve, GM’s vice president of vehicle sales, service and marketing, in prepared remarks ahead of the delivery announcement at the Washington Auto Show later today.

The Washington Metropolitan Transit Authority will receive 952 units, which GM considers potentially the largest single hybrid-bus fleet acquisition in history. The Southeastern Pennsylvania Transportation Authority purchased 480 hybrid buses and Minneapolis/St. Paul Metro Transit plans to buy another 300 units, bringing the GM-Allison fleet under operation in the U.S. to more than 2,700 buses. The orders should take about three years to fill, GM says.

Taken together, the three orders will save an estimated 2.4 million gallons (90.8 million L) of fuel annually, or enough to fill 300 tanker trucks, GM says.

The buses, which first arrived in 2003, draw power from a parallel hybrid-electric powertrain that consists of a nickel-metal-hydride battery pack, a pair of electric motors and a traditional diesel engine.

“To date, the GM-Allison hybrid system has operated successfully for more than 55 million in-service miles (88.5 million km) without an end-of-life battery failure,” LaNeve says.

There currently are about 1,000 GM-Allison hybrid buses on the road today. GM licenses the hybrid technology to its former Allison Transmission Inc. subsidiary, which assembles and sells the hybrid transmission to bus manufacturers. Later this month, GM will deliver the 1,000th GM-Allison hybrid bus to the Regional Transportation Commission of Southern Nevada.

One recent study concluded hybrid buses deliver 75% better fuel economy than traditional transit buses and reduce oxides of nitrogen emissions up to 39%. Particulate matter is trimmed 97%, carbon monoxide up to 60% and hydrocarbons upwards of 75%. All are considered greenhouses gases that scientists say contribute to global warming.

GM uses the same 2-mode hybrid technology to power its ’08 Chevrolet Tahoe and GMC Yukon fullsize SUVs and will add it to several other passenger cars by the end of the year.