Hyundai Motor Co.'s quick embrace of downsized engines, electric hybrids and other anti-pollution technology helped the company unseat Honda Motor Co. as the most environmental automaker, a scientific research group says.

Toyota Motor Corp., Nissan Motor Corp. and Volkswagen AG rounded out the top 5 in rankings released by the Union of Concerned Scientists, a nonprofit that works on climate change and nuclear-weapon control.

Ford Motor Co., General Motors and Chrysler Group were at the bottom.

Bloomberg reports that companies that sell cars in the United States are racing to meet stricter greenhouse-gas limits that begin to step up in 2017 and will require automakers to double the average fuel-efficiency of their fleets to 54.5 miles per gallon by 2025.

That has led to production of dozens of new hybrids, plug-in cars, electric vehicles and models powered by fuel-saving gasoline engines.

"While automakers have widely used a few key technologies to reduce the global warming impact of their fleets, other technologies on the horizon are also important," the Cambridge, Mass.-based group of scientists said.

It cited smaller turbocharged engines and hydrogen fuel cells. Overall, all eight of the companies that sell most cars in the United States have reduced their global-warming emissions, the group said -- the first time that's happened in six reports going back to 2000. The rankings didn't take into account automakers with smaller U.S. sales, such as Tesla Motors Inc.

Honda had won the title in each of the group's previous five assessments, which has been dominated by the three major Japanese automakers. Ford, GM and Chrysler were the only companies this year to score below the industry average.

Ford has made the most efficiency gains among the three- largest U.S.-based automakers, partly by expanding the number of hybrids it sells. Efforts by the company to boost the efficiency of "its smallest vehicles -- the Fiesta and Focus -- and its best-selling and largest vehicles, the F-series pickups, has helped bring its fleet performance to within a few percent of the national average."

General Motors, which sells the plug-in electric hybrid Chevrolet Volt, hasn't kept pace with the industry even though it has improved efficiency in its small cars, the group said. GM needs to improve its light-duty truck fleet to gain on competitors, the scientists said.

Hyundai, which also makes the Kia brand, was praised by the group for "a concerted effort" to improve fuel efficiency by turbocharging and downsizing engines, as well as for gasoline-electric versions of the Hyundai Sonata and Kia Optima.

"As Hyundai-Kia works to further improve fuel economy and electrify its fleet, Honda will need to step up its innovation if it wants to take back the crown," the scientists wrote.