For many years, Honda was routinely awarded the title of greenest carmaker in the U.S. market. But its most recent green cars have stumbled, and a dark-horse competitor is rising fast toward taking the coveted title, conferred every few years by the Union of Concerned Scientists.

It's not Nissan, which launched the 2011 Leaf battery electric car that won our GreenCarReports 2011 Best Car To Buy award. It's not GM, with its much-lauded 2011 Chevrolet Volt range-extended electric car. It's not even Toyota, which sells the uber-hybrid Prius and has built roughly two-thirds of all the hybrid-electric vehicles on the planet.

It's Hyundai.

2011 Hyundai Sonata

2011 Hyundai Sonata

Korean competitor

It tied with Toyota for second place in the UCS rankings, which scores each carmaker based on the “smog-forming and greenhouse gas emissions (primarily CO2) in its U.S. automobile fleet." Note, however, that this year's award was based on 2008 model-year data.

Last December, Hyundai claimed a different win: Its Corporate Average Fuel Economy (CAFE) for 2010 was the highest of "all major manufacturers," including "traditional leaders like Honda and Toyota." One contributing factor: Hyundai sells no pickup trucks.

2010 Hyundai Tucson

2010 Hyundai Tucson

30 mpg, fewer V-6s

Hyundai squeaked over the magic 30-mpg mark, with a CAFE rating of 30.1 mpg. The top five also included Honda at 29.7 mpg, Volkswagen at 29.6 mpg, Toyota at 29.4 mpg, and Kia (which is closely tied to Hyundai) at 28.0 mpg. The company may soon release similar data when the EPA completes ratings for all 2011 model-year vehicles.

A year ago, Hyundai startled the industry by saying it wouldn't offer V-6 engines in its midsize 2011 Sonata sedan or its compact 2011 Tucson crossover. Then, this summer, it pledged to raise its fleet average fuel economy to a remarkable 50 miles per gallon by 2025.

2011 Hundai Sonata Hybrid, La Jolla, California, October 2010

2011 Hundai Sonata Hybrid, La Jolla, California, October 2010

Adding its own hybrid

Next month, the first 2011 Hyundai Sonata Hybrid models will arrive at dealers, adding a new and significant player to the short list of makers who sell affordable hybrid-electric cars in significant volume (more than 10,000 per year, say). It is rated at 37 mpg city, 39 mpg highway, and offers electric running as high as 71 miles per hour.

Within a couple of years of the Sonata Hybrid, Hyundai is expected to launch a plug-in hybrid based on the BlueWill concept car it has displayed at auto shows over the last two years.

2010 Honda Insight - rear three-quarter

2010 Honda Insight - rear three-quarter

Honda Insight struggles

Honda, on the other hand, is widely acknowledged to have missed the mark with its 2010 Honda Insight five-door hybrid hatchback. While it was the least expensive hybrid in the U.S. on launch, its first-year sales were anemic. And its 2011 CR-Z sports hybrid is only a two-seater, making it a minor contender despite its $19,200 price.

The Insight's mild hybrid system does not allow electric-only running, which some view as an important marketing tool. And its EPA ratings of 40 mpg city, 43 mpg highway are bested by those of the larger midsize 2011 Toyota Prius (51 mpg city, 48 mpg highway), which has a base price just a couple of thousand dollars higher.

Worse yet, the Insight sits right next to the Honda Fit on showroom floors. That highly ranked subcompact has more interior space, is more fun to drive, gets mileage ratings of 28 mpg city, 35 mpg highway with a five-speed gearbox, and is several thousand dollars cheaper. Frankly, you really have to want a hybrid to pick the Insight over the Fit.

2011 Honda CR-Z

2011 Honda CR-Z

Not standing still

Honda is hardly standing still.

It unveiled a prototype of an all-electric Honda Fit EV 10 days ago at the 2010 Los Angeles Auto Show, and it is developing a new, more powerful hybrid system for larger vehicles. It is still the only carmaker to sell a natural-gas vehicle (the Honda Civic GX) in the U.S. market, and its reputation for good gas mileage is a huge marketing asset.

In volume cars, its new Civic compact sedan will launch for 2012, and it is said it to be slightly smaller in exterior size, but more capacious and far more fuel-efficient than today's model lineup.

Still, Hyundai's showing has surprised many observers. Given its aggressive expansion, and the market-share gains it captured during the auto-sales meltdown of the past two years, it seems well positioned to give Honda a run for its money.

Which can only be good for green car buyers.