Tom Magliozzi’s beloved 1952 MG Roadster just won’t fire up.

"It sounds like a sick cow," clamors his brother, Ray Magliozzi.

The car, and the two genetically tied jokesters also known as Click and Clack, have for 30 years been the stars of the National Public Radio show "Car Talk," in which listeners describe, using both words and vocal mimicry, an ailing vehicle's symptoms in exchange for a humorous diagnostic.

In the hunt to replace Tom's mechanical love, the brothers took to the road. And the journey -- really an excuse to film the brothers' slapstick efforts to uncover America's four-wheeled future and what it will take to make the country's vehicles more efficient -- became the subject of a documentary called "Car of the Future," which airs on PBS’s Nova series Tuesday night (at 8 p.m. in most places) in honor of Earth Day.

The Magliozzi brothers, who are making their television debuts, already have placed a warning on their website: "America's ugliest radio show hosts are baring their mugs, in the interest of exploring some of the intriguing automotive technologies currently on the horizon."

Their efforts take them to the glitz and glam of the North American International Auto Show in Detroit, where the boys question an auto exec about the need to produce a gas-guzzling, 500-horsepower vehicle that seems excessive even to couple of self-described “car nuts” like themselves.

On the other end of the green spectrum, the brothers squeeze into a tiny 100 miles-per-gallon three-wheeler that can't drive backward at the AltWheels Festival in Boston.

The brothers' pilgrimage also takes them to Iceland to explore hydrogen-powered cars, to Boston-based Mascoma to chat with co-founder Lee Lynd about biofuels, to the University of California at Davis to check out a plug-in hybrid with professor Andrew Frank and to San Carlos, Calif.-based Tesla Motors to drive an electric sports car and chat with founder Martin Eberhard.

Click and Clack aren't the only ones taking a lighter look at green vehicles on Earth Day. is preying on the U.S. obsession with celebrities by listing the must-have green cars of the rich and famous.

With a star-studded waiting list that includes George Clooney, Jay Leno, Matt Damon, Flea, Anthony Kiedis, Brad Pitt and Arnold Schwarzenegger and a $98,000 price tag, Tesla's sporty electric Roadster has made the grade also predicts the BMW Hydrogen 7, which has included Will Ferrell, Cameron Diaz and Edward Norton as its invite-only test drivers, and the Cadillac Escalade Hybrid, which had Super Bowl winner Eli Manning among its first customers, will be hot celeb items.

For those who are just can't get enough of celebrities frolicking alongside a green ride, has put together an Earth Day slide show, where viewers can see Paris Hilton waving from her GMC Yukon Hybrid and Snoop Dogg hanging out next to the Chevrolet Volt.

Non-celebrities who want green cars also got some news this week. Norwegian electric-car company Think Global announced Monday that it had created a joint venture with cleantech investors RockPort Capital Partners and Kleiner Perkins Caulfield & Byers to bring its cars to the United States. The newly minted company, Think North America, will aim to sell electric cars on the western side of the pond.

Think Global already has one vehicle on the European markets called Think City, which reaches a top speed of 65 mph and can drive on 110 miles on a single charge.

In October, Think Global told Greentech Media that it was looking to break into the U.S. market and planned to bring its five-seat crossover concept vehicle, called the Think Ox, to the United States as early as 2010.