Hyundai and Kia made the greenest cars last year, according to an annual ranking by the Union of Concerned Scientists.
The sister Korean companies stole the crown from Honda, which had been No. 1 since 1998.
Researcher Dave Cooke says Hyundai and Kia's strategy of using smaller, turbo-charged engines is paying off, "and they also added two hybrid electric versions of some of their most popular vehicles, the Hyundai Sonata and Kia Optima."
The ranking looks at average fuel economy and emissions of the top eight automakers in the U.S.
Ford, General Motors and Chrysler ranked in the bottom three, primarily because they make a lot more big SUVs and pickup trucks.
For the first time since 1998, all eight of the top U.S. automakers improved their cars' average fuel economy and emissions.
Cooke says it's an encouraging sign as automakers work to meet the nation's fuel economy standards.
Those standards will get stricter every year, through the year 2025.
"They have essentially turned a corner," says Cooke, "and so we expect as we move toward the 2025 standards that this is going to only continue to improve."
The report notes that each automaker is using a different strategy to improve its cars' fuel economy and emissions.
Ford, similar to Hyundai and Kia, is relying heavily on smaller, turbo-charged engines; Volkswagen, on diesel; Nissan on electric and hybrid cars.