As automakers move to produce cars that use less gas and emit fewer pollutants, foreign-based car companies are edging out American rivals. But U.S. auto makers are catching up.

That's the conclusion of a report released Thursday by the Union of Concerned Scientists, an environmental group in Cambridge, Mass., that ranks the eight "greenest" makers of cars sold in the United States, The Wall Street Journal reported

The rankings are based on the average fuel economy and smog-causing emissions of each auto maker's product lineup from the 2008 model year, with each company getting a numerical rating. The group said the 2008 model year was the latest for which government figures were available.

Japan's Honda Motor Co. won top honors as the greenest car company, narrowly beating out Japan's Toyota Motor Corp. and South Korea's Hyundai Motor Co., which tied for second. Germany's Volkswagen AG and Japan's Nissan Motor Co. Ltd. weren't far behind.

Then there were the Detroit Three automakers, which account for the majority of vehicles sold in the U.S. They took the bottom three spots on the list: Ford Motor Co., General Motors Co. and Chrysler Group LLC.

The rankings are no surprise. Car companies in Europe and Asia have for years specialized in smaller, more efficient vehicles, in large part to meet market demands in their home countries, where gas prices are higher than in the U.S. Detroit auto makers have relied on sales of larger cars, trucks and sport-utility vehicles that are more popular among Americans.

It's the fifth time in the past decade the environmental group has come out with the list, and each time, U.S. automakers have occupied the last three spots.

But there's good news this year, according to the report's authors: The U.S. companies are starting to catch up as they invest in hybrids and other fuel-saving technologies.

"The gap between the dirtiest and cleanest auto maker is narrowing," said Jim Kliesch, a UCS senior engineer and author of the report.

He said he expects all auto makers to put out more environmentally friendly cars as U.S. regulators ramp up fuel-economy standards.

The Obama administration said last week it was considering requiring that cars sold in the U.S. achieve an average of 62 miles a gallon by 2025. The administration has already mandated a 35.5 mpg average by 2016.

Several major car makers plan to introduce electric vehicles in the U.S. this year, including GM, which expects to roll out the battery-powered Volt.

Kliesch said electric cars could have a modest impact on the industry averages, at least in the short term, because he didn't expect sales to be significant.

GM, for its part, responded with a slight dig at the environmental group, and noted the company continued to improve the fuel efficiency of its fleet. "We have actual scientists and the brightest engineers hard at work on new technologies and solutions to improve fuel efficiency," spokesman Greg Martin said.