By Maria Kay Fotopoulos | Research by Karin Amour

A multitude of tumultuous national and global events during the last year — including gas prices that skyrocketed before teasingly settling back down — have brought many of us face-to-face with our profligate ways behind the wheel. The volatility and uncertainty of the future of fossil fuels has prompted rethinking around how we get from point A to point B.

Can our four-wheel choices make a real difference? With an estimated 2.5 billion vehicles on Earth’s roads by 2050, the answer, of course, is yes. If you’re one of the millions of Americans considering trading your gas-guzzler for a gas-sipper (or skipper) in 2009, then you’ve got a few big decisions ahead of you. Fortunately, some 90 vehicles on the market already achieve 30 miles per gallon, and improved technology is pushing that number higher. Here’s our roundup of the best and the brightest. (And if you need extra help, check out the Green Vehicles Guide on the EPA’s website.

Do or Diesel

While diesel vehicles suffered image problems several decades ago, today’s “clean diesels” are a completely new breed. Automakers will bring more than 20 models to the U.S. market in the next two years. We’ll have what Europeans have long been accustomed to — diesel engines that offer better fuel economy, while employing new technology that filters the smog-forming pollutants that have given diesels their bad rap in the past. More than 60 percent of vehicles in Europe run on diesel, which offers mileage gains of 25 to 40 percent over comparable gas models.

By 2010, all gas stations will be required by law to serve Ultra-Low Sulfur Diesel (ULSD), which is diesel excised of its major polluting component, sulfur. And of course, all diesel vehicles can run on biodiesel fuel — increasingly available at pumps across the nation — with absolutely no conversion necessary. Just make sure your biodiesel provider is hip to sourcing fuel that has been manufactured responsibly, i.e., without disturbing the food cycle or contributing to deforestation in developing nations — algae-mobiles, anyone?

For the performance driver with a heart of green: 2009 BMW 335d
BMW AG prides itself on having reduced CO2 emissions more than any other carmaker in recent years. So they must be particularly proud to present the BMW 335d — the most fuel-efficient car they’ve ever offered to the United States — which goes for a premium over the gasoline-powered 3 Series. A front-engine, rear-wheel drive, five-passenger sports sedan, the BMW 335d features the award-winning sequential-turbo 3.0 liter diesel engine. It delivers the performance enthusiasts expect from a BMW (0 to 60 is 6 seconds) and a driving range of 570 miles to the next fill-up.

By the Numbers
23 city | 36 hwy
Six-speed automatic transmission
Starting at $44,725
Extra eco-credit: BMW injects urea, a compound found in urine that can be synthesized from inorganic materials, into the exhaust stream to help “neutralize” tailpipe emissions into harmless water and nitrogen. The whole thing sounds much better when it’s called “BluePerformance.”

A green alternative with no concessions: 2009 Volkswagen Jetta TDI
One of Volkswagen’s “Clean Diesels in the USA,” the 2009 Jetta TDI offers high fuel efficiency, extremely low emissions and maximum performance. The Jetta TDI is quiet — an indicator of diesel technology advances — and it’s up to 35 percent more fuel efficient than similarly powered gasoline engines. This five-passenger family sedan is eligible for a $1,300 tax credit.

By the Numbers
30 mpg city | 41 mpg hwy
Six-speed manual transmission
Starting at $21,990
Extra eco-credit: Averaging 58.82 mpg on travels throughout the U.S., this VW set a Guinness World Record for “Lowest Fuel Consumption,” prompting Green Car Journal to name it “2009 Green Car of the Year.”