Lexus RX 400h

Though sales of the gas/electric-powered Lexus RX 400h slipped by 14.2 percent last year, it remains the volume leader among both luxury and SUV hybrids, according to CNW Marketing Research in Bandon, Ore. It's also the luxury vehicle with the lowest exhaust emissions sold in the U.S. for 2008. It's even quicker than the conventionally powered RX 350 on which it's based, yet gets better fuel economy at 27 mpg city/24 mpg highway. We expect that Lexus, which scored the top three spots on this ranking simply by virtue of being the only luxury manufacturer to offer hybrids, will have to defend its green title as other high-end automakers gear up to produce hybrids of their own.

Lexus GS 450h

The Lexus GS 450h's V6 engine/electric-motor mash-up produces the equivalent of 340 hp, which makes this midsize hybrid sport sedan about as fast as a Porsche 911 Carrera when accelerating from zero to 60 mph (5 seconds). Yet it gets an estimated 22 mpg city/25 mpg highway. As the top three vehicles on this list indicated, hybrids do a great job of reducing exhaust emissions because their gasoline engines can shut down in stop-and-go traffic and, in some cases, such as with the GS 450h, can stay dormant up to speeds of about 35 mph to let the emissions-free electric motor do all the work.

Lexus LS 600h L

At over $100,000, the Lexus LS 600h L is hardly a rational automotive choice, even if it is one of the cleanest-running luxury cars on the road. Lexus claims its V8-based hybrid powertrain accelerates like a conventional V12 engine but returns class-leading fuel economy, estimated at 20 mpg city/22 mpg highway. It's not as quick as a true V12, however, and would be more economical if it used a smaller engine, but its hybrid badge still gives it cachet.

Audi TT

The base version of Audi's TT coupe and convertible comes with the 2.0-liter four-cylinder engine also offered on the A3 and A4. But the TT 2.0T is the smallest and lightest of the three vehicles, so it gets the best mileage and therefore the lowest carbon footprint, which is why it ranks ahead of the two other Audi's on this list. The EPA estimates 23 mpg city/31 highway for the TT 2.0T. Like the V6-equipped A3 and A4, the pricier TT 3.2, which comes with a mileage-sapping V6 and all-wheel drive, doesn't make our list, due to a carbon footprint of 9.2 to 9.6 tons of greenhouse-gas emissions resulting from poorer fuel economy (17-18 mpg city/24 mpg highway).

Audi A3

The front-wheel-drive A3 2.0T has the same 2.0-liter four-cylinder engine as the Audi A4 2.0T, but it gets slightly better fuel economy in city driving, where the worst exhaust is emitted, and therefore has a smaller carbon footprint than the A4. The A3 2.0T with the S Tronic auto/manual transmission, which is the model that made our ranking, gets 22 mpg city/29 mpg highway. When equipped with the manual six-speed transmission, the A3 2.0T returns 21 mpg city/29 mpg highway and a higher carbon footprint of 7.7 annual tons of greenhouse-gas emissions. As with the A4, opting for the V6 engine, which comes with all-wheel drive, increases fuel consumption and exhaust emissions.

Audi A4 2.0T

The Audi A4 2.0T is the first of three Audi models to make this list, all of which use a 2.0-liter turbocharged four-cylinder engine and two-wheel-drive configuration. The EPA estimates 21 mpg city/30 mpg highway for the A4 2.0T with the continuously variable transmission (CVT) and 20 mpg city/31 mpg highway with the manual six-speed. Both get the same air pollution and carbon footprint ratings. The optional V6 engine offered on the A4 also gets an Air Pollution score of 6 but consumes more fuel and therefore has a larger carbon footprint, which ranges from 8.7 to 9.6 annual tons of greenhouse-gas emissions, depending on the transmission and whether it is paired with all-wheel drive.

Porsche Boxster/Cayman

You might not expect sports cars to land on any list of nature-friendly autos, but the Porsche Boxster roadster and Cayman coupe qualify by virtue of their fuel-efficient engines. At 20 mpg city/29 mpg highway with the available five-speed manual transmission, the cars' 2.7-liter six-cylinder engines deliver economy-car efficiency with true Porsche muscle.

Volvo S40 2.4i/V50 2.4i

The base five-cylinder-powered Volvo S40 sedan and its station-wagon equivalent, the V50, aren't as fast or sporty as other premium compact cars, but their smaller, less-powerful engine represents a gain for the environment. If you live in California or the other states that adhere to its stricter emissions regulations, you can drive an S40 or V50 that's given an Air Pollution Score of 9.5, while those in the rest of the U.S. settle for a version rated at 7, which is still above average. We used the latter score for this ranking because it corresponds to the vehicles most widely available. The 2.4i variants come only with front-wheel drive, unlike the more powerful and slightly less environmentally friendly T5 models not on this list, which can be ordered with all-wheel drive.

Lexus ES 350/GS 350

The midsize Lexus ES 350 and sportier GS 350 boast good fuel economy (19 mpg city/27 mpg highway) with low emissions for midsize luxury sedans. Both are powered by a 3.5-liter V6 engine mated to a six-speed automatic transmission. The ES 350 is elegant, understated, and offers a solid, stable ride with front-wheel drive. By contrast, the rear-drive GS 350 is more expressive and sportier. Optional all-wheel drive on the GS 350 comes at the expense of lower fuel economy and added CO2 emissions.

Lincoln MKZ

Lincoln's smallest sedan gets 22 mpg in combined city/highway driving from its 3.5-liter V6 engine and six-speed automatic transmission. The front-wheel-drive midsize MKZ puts 4.8 tons fewer greenhouse gases on average into the atmosphere each year than does the automaker's largest model, the full-size Navigator SUV. An all-wheel-drive version is offered, but its lower fuel economy (19 mpg combined city/highway) means it will spew an extra 1.3 tons of CO2 each year compared to the standard front-wheel-drive version.

Volvo S60

The midsize Volvo S60 sedan comes powered in its base, front-wheel-drive version by an efficient and clean-operating 2.5-liter five-cylinder turbocharged engine that delivers peppy, though not overpowering, acceleration. Estimated fuel economy is 19 mpg city/27 mpg highway. An all-wheel-drive version of the base model and a quicker T5 model with a 2.4-liter turbo five-cylinder engine are also offered, but both get lower fuel economy, and have higher carbon footprints.

Mercedes-Benz C300

The base model of Mercedes-Benz's smallest sedan uses a 3.0-liter V6 engine. Choosing the six-speed manual transmission and standard rear-wheel drive, rather than the available automatic transmission or all-wheel drive, offers the best fuel economy of the C-Class lineup. At an estimated 18 mpg city/26 mpg highway, the Mercedes C300's fuel economy is barely better than the Infiniti G37 Coupe's. But that one extra mpg in stop-and-go traffic makes a significant difference for its carbon footprint, which is noticeably smaller than the Infiniti's. The more powerful C350 isn't as kind to the environment as the C300.

Infiniti G37 Coupe