Honda and Toyota left Detroit’s Big Three trailing in the dust in the Union of Concerned Scientists’ (UCS) biennial ranking of the greenest automakers.

For the fourth time in a row, Honda topped the science organisation’s rankings as the country’s greenest automaker. Toyota followed closely behind having made significant cuts in global warming pollution. Hyundai-Kia, Nissan and Volkswagen were in the middle of the pack, coming in third, fourth and fifth, respectively. Ford and General Motors, meanwhile, were at the back, and DaimlerChrysler placed last.

“There is a huge gap between the cleanest and dirtiest automakers,” said Don MacKenzie, author of the report and a vehicles engineer with UCS. “The winners are using clean technology across their entire fleets. The losers are installing it piecemeal, or not at all.”

MacKenzie analysed the performance of ten classes of vehicles produced by the eight automakers which comprise 96 percent of the US car and light truck market in model year 2005. Each automaker was rated on how its vehicles compared to the industry average on global warming and smog-forming pollution. Cars and light trucks account for 25 percent of the nation’s global warming pollution and 20 percent of its smog-forming pollution.

Honda and Toyota had better-than-average global warming scores in every class in which they competed. Despite producing pickup trucks and large SUVs, Toyota’s use of emissions-cutting technology across its entire fleet helped it to pull up just behind Honda, which did not compete in these vehicle classes.

“Toyota’s ranking shows that size is no excuse for a dirty fleet,” MacKenzie said. “All of the automakers have the technology today to make all of their vehicles, from two-seaters to four-by-fours, a lot cleaner. And given the Supreme Court ruling confirming carbon dioxide and other global warming emissions are pollutants, it’s likely that federal or state efforts will succeed in requiring automakers to put that technology to work.”

Ford was the cleanest of the Detroit automakers, finishing sixth. But if Ford had made the same progress cutting global warming pollution in its U.S. fleet as it has with its European fleet, it would have finished in fifth.

GM, which placed last in UCS’s 2003 model rankings, moved past DaimlerChrysler by reducing its fleet’s smog-forming emissions. But the country’s largest automaker failed to improve its global warming pollution score since the last UCS automaker rankings. In model year 2005, GM had the dubious distinction of selling the most vehicles rated at 15 miles per gallon or worse in city driving.

DaimlerChrysler came in dead last with the worst scores for both smog and global warming pollution. Its cars and trucks emit 70% more smog-forming pollutants and nearly 30% more global warming pollutants per mile than those made by Honda.

The reports authors said that the top two finishers, Honda and Toyota, are the only two automakers with consistently better-than-average performance in both the smog and global warming categories in nearly every vehicle class. In contrast, GM undermines its leadership in some vehicle classes with poor performance elsewhere.

“Americans are paying closer attention to their personal environmental impact, and they want greener cars,” said Ted Grozier, an associate at the environmental strategy consulting firm GreenOrder. “The successful automaker is going to figure out a way to deliver those cars to consumers.”

The UCS analysis showed hybrids helped improve environmental performance while diesels generally held automakers back. Volkswagen’s diesel engines, for example, slightly improved its global warming score, but significantly dragged down its smog score. Hybrids, meanwhile, helped Toyota cut its global warming pollution fleetwide because the company produced them in large numbers. Honda and Ford, which produced fewer hybrids, did not see the same improvement.