Honda has always been a leader in the realm of fuel efficiency and environmentally friendliness. The Japanese auto manufacturer has consistently rolled out Ultra-Low Emissions Vehicles (ULEVs) and Super Ultra Low Emissions Vehicle (SULEVs) that dump less polluting emissions into our atmosphere. Honda brought the first hybrid-electric vehicle to the U.S. market in the form of the Insight. The tiny, tadpole-esque two-seater weighed less than 1,900 pounds and managed to achieve EPA mileage ratings of 60MPG/66MPG city/highway with a manual transmission.

Honda was also one of the first auto manufacturers to reintroduce the use of continuously variable transmissions (CVTs) to the North American market in the mid 1990s with the Civic HX -- Subaru had first tried out CVTs in the 1980s with the Justy. CVTs allow the engine to run at the most efficient RPMs and allow for increased fuel efficiency. Likewise, Honda has resisted the urge to drop a potent V8 engine in its largest SUVs and luxury sedans and has instead relied on pushing its efficient 3.5 liter V6 engine family to customers who purchase its largest vehicles.

This level of restraint and eco-friendliness on the part of Honda has led it to be labeled as the "2007 Greenest Automaker" by the Union of Concerned Scientists (UCS). This is the fourth consecutive year that the automaker has won the award.

The top 8 auto manufacturers -- which represent 96% of the U.S. car and light truck market -- were tested across ten MY2005 vehicle classes on tailpipe emissions and overall contribution to global warming. You can download the full results of the UCS test here (PDF).

"Honda remains the greenest U.S. automaker. The company installs clean technology across its entire fleet of cars and trucks and that consistency makes it a top environmental performer" said Don MacKenzie, a vehicle engineer for the UCS and author of the report. "In addition, Honda continues to have the best smog score in four out of the five classes."

Honda slightly beat out second place Toyota which has also made strides to cut emissions and improve fuel economy across its entire vehicle lineup. "Toyota's ranking shows that size is no excuse for a dirty fleet," MacKenzie continued. "All of the automakers have the technology today to make all of their vehicles, from two-seaters to four-by-fours, a lot cleaner."

Hyundai/Kia placed third with Nissan and Volkswagen taking fourth and fifth respectively. Ford, General Motors and DaimlerChrysler rounded out the tail end of the list. General Motors was singled out for having the most vehicles in the lineup with EPA city mileage ratings of 15MPG or lower. Last place DaimlerChrysler was also criticized for its fleet of vehicles which produce 70% more pollutants than first place Honda and earned the "Rusty Tailpipe Award."

"There is a huge gap between the cleanest and dirtiest automakers," MacKenzie. "The winners are using clean technology across their entire fleets. The losers are installing it piecemeal, or not at all."

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

Just last week, President Bush issued a call to auto manufacturers to boost fuel efficiency and reduce greenhouse gas emissions in new vehicles. Bush's plan calls for a 20% reduction in gasoline usage by 2017 and a halt in the rise of greenhouse emissions. The move is expected to cost the auto industry $114 billion USD.