You’ve heard about the “cash for clunkers” legislation that allows U.S. car buyers to get up to $4,500 in cash vouchers for replacing gas guzzlers with fuel efficient vehicles. If you’ve got an older car or pickup truck with rotten gas mileage, you can replace it with a new car getting 22 mpg or better or a new pickup getting 18 mpg or better and receive a break on the price courtesy of your government.

Which cars should you be looking at? Marketing service Autobytel complied a list of auto makers that have vehicles that qualify for vouchers, and even figured out how many of the cars in each automakers fleet were eligible. Every vehicle offered by Mini and Scion, for instance, could get you some cash toward a new car if you trade in a clunker.

Here’s Autobytel’s top ten cash for clunkers cars, with the percentage of each maker’s fleet that qualifies for a voucher when you buy a new car:

1. Mini 100%
2. Scion 100%
3. Honda 94%
4. Saturn 93%
5. Subaru 88%
6. Suzuki 85%
7. Pontiac 79%
8. Mazda 74%
9. Hyundai 73%
10. Volkswagen 73%

Honda doesn’t receive perfect marks probably because it has that massive Ridgeline pickup truck in the fleet. And Volkswagen all the way down at number 10 was a surprise. Also, keep in mind that Saturn’s future is a bit shaky these days, and Pontiac has received an official death sentence as part of GM’s restructuring.

The latest online app to help drivers track fuel economy comes from Moblu. The site asks for your vehicle’s year, make and model then requires you to create a user name and password.

Where, you might be asking, does the social networking come in? Moblu uses your Twitter user name (no Twitter password required) to share your mileage with other Tweeple. Moblu users can record events that might affect gas mileage, like “Changed Tire Pressure” or “Lots of Highway Driving.” You can also record a fill-up, calculate mileage for a road trip, and find fellow Moblu users on the site.

Clicking on “My Garage” shows fellow Moblu cars, with actual mileage, trends, and number of fill-ups. You can choose to follow anyone who looks interesting, or keep tabs on your friends’ fuel economy.

The National Renewable Energy Laboratory have set their sights on one of the least efficient systems in modern automobiles: the air conditioning. The laboratory says 6% of the total amount of fuel consumed in the U.S. — about seven billion gallons, if you can imagine — is used to run the a/c.  The EPA piles on the bad news by reporting that leaky automotive air conditioning units pump 50 million metric tons of CO2 into the air each year.

That’s where the NREL is hoping to make a difference. According to a post on the New York Times’ “Wheels” blog, engineers are hoping to improve the a/c in cars by as much as a third. If they succeed, it could be a big step toward the new CAFE standards, which require an average fuel efficiency of 35.5 mpg by 2016.

As part of the German carmaker’s promotional effort for its clean diesel engines, Audi will donate $1 to the Nature Conservancy’s carbon offset program for every Facebook user who joins the program, up to $25,000.

Audi TDI clean diesels reduce carbon dioxide emissions by up to 20% and increase fuel efficiency by up to 30% over gasoline-powered cars, according to the company. “Audi’s contribution will provide necessary support to our voluntary carbon offset program’s reforestation and forest protection projects, but it will also help us to bring the benefits of the project to a new audience of supporters,” said Zoe Kant of the Nature Conservancy in a press release.

If you’re a Facebook user and you want to take part in the program to offset your own carbon emissions and get that extra $1 donation per person from Audi, visit www.causes.com/natureconservancy.

Remember when I said diesel prices were lower than gasoline for the first time in years? Remember when I wondered if that would change American diesel buying habits? Well, it has, apparently. Volkswagen and Mercedes-Benz are both reporting increases in sales of their clean-diesel vehicles.

If you’re in the market for a clean diesel car, here are the 50-state diesels available right now, with their starting MSRP:

• Audi Q7, SUV, $50,900
• Mercedes R320 BlueTec, crossover, $49,100
• Mercedes ML320 BlueTec, SUV, $48,600
• Mercedes GL320 BlueTec, SUV, $58,200
• Volkswagen Jetta TDI, sedan, $22,700
• Volkswagen Jetta SportWagen TDI, wagon, $23,870
• Volkwsagen Touareg 2 TDI, SUV, $42,800

Each of these is available from the manufacturer as of today, and are eligible of a federal tax credit of about $1000. Check FuelEconomy.gov for details.

Image of a BMW diesel at the 2009 Detroit Auto Show by Kristen Hall-Geisler.

The Ohio State Highway Patrol is outfitting its fleet of Ford Crown Victorias with solar power. The $37 panels are much smaller than the rooftop array found on, say, the 2010 Toyota Prius or the forthcoming Fisker Karma. The goal is to reduce the time that cops spend idling with the engine on by powering some of their gear with energy from the sun.

The panel is wired directly to the car’s battery, which would normally power the police gadgets when the car is turned off. To extend the battery’s run time, and therefore the amount of time the engine is running, the solar panel will add clean, zero-emissions juice to the system.

The panels, according to Edmunds.com, are mounted inside the rear window (ever leave a chocolate bar back there? A proven sun-catching site). The brackets used to hold the panels are green, too: they’re recycled from misprinted license plates.

Hi-def TVs, iPods, digital picture frames … and now, electric two-wheelers. Best Buy announced last week that it would add plug-in bikes and motorcycles to its stores, starting with the Brammo Enertia in Portland, Oregon.

The Oregonian’s Hard Drive blog, the Portland Business Journal, Good Morning America, and the New York Times all reported on the sale of the bikes, which also include models from Currie Technologies and Ultra Motors.

The vehicles will be zero emissions and capable of commuting. The Enertia, for example, has a top speed of 55 mph and costs $12,000 before state or federal tax credits.

We already knew that the Mitsubishi i-MiEV electric car would go on sale in July 2009, but we didn’t know how much it would sell for. I, for one, would never have guessed that the price tag would be a whopping $45,660. There’s a lot of new tech packed into that teensy package.

Other high-tech electric cars are expected to reach the $40,000, including the anticipated Chevy Volt and the already announced Subaru Stella, which starts at $49,255, according to the New York Times’ Wheels blog.

What other cars cost about $50,000? Let’s take a look:

• 2010 Lexus RX Hybrid: $41,660
• Mercedes-Benz ML320 BlueTec clean diesel: $48,600
• Volkswagen Touareg 2 TDI clean diesel: $42,800

But all of these, for their steps in a green direction, require fossil fuels in one form or another and emit CO2 to a greater or lesser degree. The i-MiEV will be all-electric and zero-emissions.

Image of the Mitsubishi i-MiEV by Kristen Hall-Geisler.

So say the geniuses over at Popular Science, anyway, and who am I to disagree? Each year, more than 95% of the 10 million vehicles headed for the parking garage in the sky end up being recycled. Of those cars, 75% of the parts are recycled. Thus, the crown for world’s most recycled product.

Here’s how PopSci breaks it down:

• 65% of the average vehicle is metal, like steel, iron, or copper, which can be used in construction
• 90% of all lead-acid batteries are recycled
• 86% of rubber tires are used to make pavement or fuel paper mills
• 380 million gallons of oil are refined and reused

Michelin has announced its new tire, the Energy Saver A/S, calling it “the most fuel-efficient tire on the road.” It promises up to 8% better fuel economy and reduced carbon dioxide emissions for any car that uses the tire, plus Michelin says its even offers better braking capabilities than other tires.

Here’s the Energy Saver’s claims to fame:

• New construction and tread rubber keep the tire cooler, delivering the 8% increase in mpgs
• Stops 8 feet shorter than similar tires
• Longer tread life means fewer tires get tossed in the garbage
• Available for conventional and hybrid cars

The tires are in the $140-$150 range per tire, depending on the size required for your car. Definitely not cheap, but if you can swing it, they may save on gas in the long run in addition to helping reduce CO2 emissions.