Honda has long had the reputation of offering fuel-efficient, relatively green vehicles--but this year, it has lost the "Greenest Automaker" laurels that it's held since 1998 to an upstart: Hyundai-Kia.

The latest award, detailed in a report that's now been issued six times by the Union of Concerned Scientists (UCS), rates the environmental performance of 2013 model-year vehicles from the eight largest carmakers selling in the U.S. (meaning Tesla Motors wasn't included).

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And the news is remarkably good: For the first time since 1998, every one of the eight automakers has lower average carbon and smog-forming emissions than the baseline numbers established that year--more than a decade and a half ago.

2014 Honda Insight

2014 Honda Insight

Average climate-change gas emissions from the average vehicle sold in the U.S. are down by nearly 20 percent over the 1998 number, according to the UCS data, while much stronger tailpipe emissions standards have reduced smog-forming emissions by a remarkable 87 percent since 2000.

Credit those reductions to stronger rules that raise corporate average fuel economy levels every year from 2012 through 2025, reducing emissions accordingly.

Hyundai-Kia was able to topple Honda through a "concerted effort to improve the green performance of its fleet by turbocharging and downsizing engines in a number of its models," as well as launching--and then quickly updating--both the Hyundai Sonata Hybrid and the Kia Optima Hybrid mid-size sedans.

FOR MORE: 2014 Kia Optima Hybrid Updated: Aerodynamic Tweaks And More

Honda has long offered mild hybrid powertrains its small cars, but the sales of those models since 2010 have been far lower than predicted by Honda.

The UCS notes that it used EPA fuel-efficiency ratings for Hyundai and Kia models that had been adjusted downward after the EPA and the companies reached a settlement over testing-procedure errors that led to overly high ratings not delivered in real-world use.

2014 Kia Optima Hybrid

2014 Kia Optima Hybrid

Small numbers of the hydrogen-fueled 2015 Hyundai Tucson Fuel Cell will go on sale later this year, as will the battery-electric 2015 Kia Soul EV.

Honda came in second to Hyundai-Kia, but it has "started to lag the industry average in its midsize fleet," said Dave Cooke, a vehicles analyst in the Clean Vehicles Program and the author of the UCS report.

Timing is partly to blame for that; the redesigned 2013 Honda Accord has been well reviewed, but despite a new continuously variable transmission (CVT), its fuel efficiency is not notably better than competitors.

Meanwhile, the Honda Accord Hybrid model that earned the Green Car Reports 2014 Best Car To Buy award is designated a 2014 model--so was not included in the report.

ALSO SEE: Green Car Reports' Best Car To Buy: 2014 Honda Accord Hybrid

That model, rated at 47 mpg combined, continues to be in short supply as Honda works to ramp up production to meet consumer demand.

Three automakers tied for third place: Nissan, Toyota, and Volkswagen. Every import manufacturer did better than the overall average, but among the three domestic makers, the UCS report cited Ford, which had the highest percentage reduction in smog-forming emissions among the eight.

This year's Automaker Rankings report from the Union of Concerned Scientists can be downloaded for more details.


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