With its EPA-sanctioned fuel economy numbers now released, we can compute an AutoEcoRating for the new, all-electric Nissan Leaf. The starting point is the Leaf’s combined city-highway fuel economy rating of 99 miles per gallon (MPG) of gasoline equivalent. That’s the number featured on its window sticker and based on EPA’s energy-equivalency formula, corresponding to an average electricity use of 34 kilowatt hours per 100 miles.

Those watt hours aren’t pollution free, and tallying up the greenhouse gases and other emissions from generating electricity with a U.S. average mix of power plants (about half coal) results in an AutoEcoRating of 195. For perspective, the average new vehicle (car-light truck average) sold this past year gets a rating of 95. The Leaf’s rating is essentially double that  — clearly a large leap in planetary politeness.

Note, however, that it’s not as big a benefit as it might seem if you only went by the fuel economy numbers. EPA reports an average combined city-highway value of 27 MPG for the average compact. The Leaf’s 99 MPG is more than triple that value. But counting the pollution from generating the power (and some from building the car) makes for a more complete comparison. AutoEcoRating takes all of these factors into account. Using the lifecycle-based environmental impact scale that underpins the eRating, the Nissan Leaf is half as polluting than the average new light vehicle sold in the U.S. market this past year.

2011 Nissan Leaf

With a waiting list of buyers already lined up, the Leaf goes on sale and into customers’ hands this month. Nissan’s initial markets for the Leaf will be mostly in the west — California, Arizona, Oregon and Washington state. They’ll also deliver some in Tennessee, home of Nissan’s North American headquarters and a planned location for Nissan manufacturing of both electric cars and their advanced batteries starting in 2012. The models of the Leaf now reaching the market are produced in Oppama, Japan, a city just south of Tokyo where Nissan has long had some of its main factories.

Nissan is positioning the Leaf as the world’s first mass-market electric vehicle (EV). The Tesla Roadster beat it to this “new era” EV market, but at a much higher price. After factoring in a $7,500 tax credit, the Leaf will cost about $25,000, and perhaps even less depending on where you buy it and whatever state incentives are available.

All of the major automakers had limited EV offerings over ten years ago in response to California’s zero-emission vehicle (ZEV) mandate. Those early electric cars were indeed much greener than any gasoline car at the time. Even though today’s gasoline  vehicles are a good bit cleaner at the tailpipe, EVs still have a clear edge for overall eco-friendliness. Nissan is banking on the Leaf to establish its leadership in electric cars — or “zero-emission vehicles” as they’re not quite correctly called. Although zero at the tailpipe doesn’t mean zero at the smokestack, the Leaf is still quite the clean machine as its 195 AutoEcoRating shows.