It might be better to keep the one you have. The American Council for an Energy Efficient Economy (ACEEE) a non-profit research group, has rated their top thirteen "green" cars. These are the least environmentally offensive models from mainstream automobile manufacturers. Non-traditional vehicles like the Twike and the NmG (the old Sparrow) are not included.

I was curious how much a green car would cost in dollars, so I used Edmunds' True Cost to Own value and and their Gas-Guzzler calculator. I entered a 2000 Mercury Grand Marquis, more or less what my father still owns, as the guzzler. I used $4.00/gallon as the fuel price. The results are interesting:

ACEEE Score, Vehicle Name (fuel) True Cost to Own - Monthly savings - Break even

54, Honda Civic GX (natural gas)  $NA - Save $79/month - 292 months      
54, Nissan Leaf (electric batteries)  $30,649 - Save $182/month - 177 months
53, SmartForTwo Coupe (gasoline)  $26,609 - Save $111/month - 117 months
52, Toyota Prius I (gas/electric hybrid)  $32,586 - Save $142/month - 144 months
51, Honda Civic (mild gas/electric hybrid)  $32,561 - Save $125/month - 173 months 
50, Honda Insight (mild gas/electric hybrid)  $28,173 - Save $125/month - 130 months   
50, Ford Fiesta SFE (gasoline)  $28,390 - Save $97/month - 138 months
49, Chevy Cruze Eco (gasoline)  $33,294 - Save $101/month - 163 months
49, Hyundai Elantra (gasoline)  $30,312 - Save $ 101/month - 152 months
49, MINI Cooper (gasoline)  $31,391 - Save $ 97/month - 171 months
49, Toyota Yaris (gasoline)  $29,256 - Save $ 93/month - 128 months
48, Mazda 2 (gasoline)  $24,822 - Save $ 97/month - 127 months
48, Chevy Volt (electric/gas hybrid)  $39,142 - Save $ 179/month - 216 months

The fuel cost savings are attractive, but Edmunds' break even number is daunting. Basically it tells me that running your old guzzler into the ground is still a lot cheaper than buying a new car. As noted in Wired's Autopia back in 2008, from an environmental standpoint, there's a lot to be said for exploiting the embedded environmental costs in an older car as well.

Ditching your gas guzzler is a great way to reduce your carbon footprint, but if you really want to do something about global warming, get a used car. You’ll be putting less carbon dioxide into the atmosphere.

As Matt Power notes in this month’s issue of Wired, hybrids get great gas mileage but it takes 113 million BTUs of energy to make a Toyota Prius. Because there are about 113,000 BTUs of energy in a gallon of gasoline, the Prius has consumed the equivalent of 1,000 gallons of gasoline before it reaches the showroom. Think of it as a carbon debt — one you won’t pay off until the Prius has turned over 46,000 miles or so.

There’s an easy way to avoid that debt — buy a used car. The debt has already been paid. But not just any used car will do.

It has to be something fuel efficient. Like, say, a 1998 Toyota Tercel that gets 27 mpg city / 35 mpg highway miles. The Prius will have to go 100,000 miles to achieve the same carbon savings as the 10-year-old Tercel. Get behind the wheel of a 1994 Geo Metro XFi, which matches the Prius’ 46 mpg, and the Prius would never close the carbon gap, Power writes.

Of course that was before the recession, Cash for Clunkers and Fukushima. As of right now, good used cars are expensive. In fact, Jalopnik looks at Kelley Blue Book values and argues, Don’t buy a used car right now :

... a surge of demand means prices for popular used vehicles are poppin'. Recent data from Kelley Blue Book indicates that so far this year, fuel-efficient segments have increased far more aggressively than they have during the past two years. Today, values of high fuel economy vehicles are up nearly 20% since January, a far cry from the steady depreciation of 2009 and 2010. At the segment level, values are up around $1,500-$2,500, with some models surpassing their respective segment average.

Examples of this increase include the Toyota Prius hybrid, which has increased in value nearly $3,800 since January 1st, and the Ford Fusion mid-size, up a substantial $1,800. As fuel prices continue to rise, Kelley Blue Book tells us they expect values for many of these vehicles to continue to increase. Makes sense, right?

But, although fuel-efficient segments have performed better than others due to the substantial rise in gas prices since the beginning of the year; it is safe to say all used vehicle values have seen spikes. Not only in dollars measured since the beginning of the year, but also in comparison to the past several years. ...

This strength in values can be attributed to a sustained lack of supply of used vehicles over the past several years, primarily stemming from reduced new-car sales through the same period, combined with the Cash for Clunkers program of two years ago providing a readily-accessible fleet of older vehicles to comb through.

So, as sales have remained low due to the economic downturn, used-vehicle supplies have been hard-hit, driving prices up over the past several years.