This week, the American Council for an Energy-Efficient Economy (ACEEE) released environmental rankings for all model year 2009 vehicles as part of ACEEE’s Green Book® Online. Vehicles are ranked according to a “Green Score,” a comprehensive measure that incorporates fuel economy, greenhouse gas emissions and tailpipe emissions.

Despite volatile conditions for the American automotive industry, this year’s rankings showcase the fact that automakers are clearly giving priority to fuel economy and green technology. Faced with fluctuating gas prices and stricter fuel economy standards, manufacturers have no choice but to offer a wider range of green vehicles by utilizing hybrid and other various efficiency technologies in vehicle design.

As a result, the 2009 “Greenest” list is an even mix of alternative motor vehicles and conventional cars with impressive emissions control and high fuel economy. Leading the pack for 2009 is the Honda Civic GX natural-gas vehicle. The Toyota Prius and Honda Civic Hybrid claim spots two and three, while the Smart Fortwo Convertible and the Toyota Yaris round out the top 5. This year’s “Greenest” list was marked by the appearance of a number of high-performing domestic cars, highlighting improved vehicle performance amongst U.S vehicle manufacturers. The Chevrolet Cobalt / Pontiac G5 twins and the Chevrolet Aveo placed 8th and 11th, respectively.

However, the big story this year is the appearance of a new crop of light-duty “clean” diesels on the U.S. market. Volkswagen, Mercedes-Benz and BMW have all released diesel, cars, SUVs and wagons this year that are available nationwide and meet emissions standards in all 50 states. In fact, the Volkswagen Jetta and Jetta Sportwagen TDI scored so well this year, they marginally missed a spot among the “Greenest” vehicles.

The “Meanest” list this year is a who’s-who of oversize pick-up trucks, SUVs and European sports cars with massive engines and poor fuel economy. However, for the first time in a while, the list is completely devoid of heavy, diesel-powered vehicles, perhaps indicating a shift in manufacture attention towards “clean” diesel technology. As a result, 2009’s worst-performing vehicles score significantly better than 2008’s “Meanest.”

With the current array of green vehicles on the market, consumers have been given a real choice when it comes to choosing an environment-friendly vehicle that suits their needs. Despite more tough times ahead, the industry is likely to continue introducing more green technologies and, consequently, a greater range of alternative and conventional vehicles that will enable the “greening” of the U.S. automotive fleet.