WASHINGTON — If motorists used rechargeable "plug-in" hybrid-electric vehicles in large numbers, the U.S. could see a significant drop in greenhouse gas emissions by the middle of the century, according to a new study.

Researchers estimated that with a market share of about 60 percent or more plug-ins, the vehicles could help reduce approximately 495 million tons in greenhouse gas emissions a year by 2050. The reductions would be the equivalent of removing 82 million passenger cars, or about one-third of the cars currently on the road.

The study, released Thursday, was conducted by the Electric Power Research Institute, a nonprofit research group, and the Natural Resources Defense Council, an environmental group. It was based on an analysis of data from the federal Energy Information Agency and EPRI.

"Plug-in hybrids are a major solution to the climate change crisis that we're facing, and the electric utility industry is indeed capable of taking over a large section of the fueling transportation sector without adding significant new capacity," said John Duncan, of Texas-based Austin Energy.

Researchers said a significant increase in plug-ins would lead to only a minor increase in demand for electricity. An increase of 7 percent to 8 percent of electric use would reduce nearly 4 million barrels of oil per day by 2050, said Mark Duvall, program manager with the Electric Power Research Institute and one of the study's authors.

The study estimated that with a more limited use of plug-ins, or about 20 percent of the market, the vehicles could remove approximately 198 million tons of greenhouse gas emissions a year by 2050.

A separate study by the organizations found that plug-in vehicles also could lead to small improvements in the nation's air quality. Most regions of the country would see improvements in ambient air quality and the reduction of pollutants, they found.

Plug-in hybrids are being developed by several automakers, including General Motors, Ford, DaimlerChrysler and Toyota. They typically feature batteries that power an electric motor with an internal combustion engine used when the batteries run low.

Owners plug the batteries into a standard wall outlet to recharge it, typically at night. The study assumed that three-quarters of the charging would take place at night during the off-peak hours of the electric grid.