NEW YORK (CNNMoney) -- The Chevrolet Volt didn't rank as one of the top-ten "greenest" cars in America, coming at no. 13, according to the American Council for an Energy-Efficient Economy's annual list.
Vehicles are ranked according to a "Green Score," factoring fuel economy and emissions, including emissions from electric power plants. The relative impact of a vehicle's manufacture and disposal, based on the car's weight, was also considered.
The top 13 cars (See correction) include gasoline-powered cars, plug-in cars, gasoline/electric hybrids and natural gas powered vehicles. No diesel cars made the top ranks because of their relatively high emissions compared to very fuel-efficient gasoline-powered cars.
The Volt can run on electricity or gasoline. For the purposes of these rankings, the ACEEE assumed the Volt was driven on plug-in electricity 64% percent of the time and on gasoline 36% of the time. The ratio was based on a standard recommended by the Society of Automotive Engineers, an ACEEE spokeswoman said.
"As a gasoline vehicle, the fuel economy's not stellar," said Shruti Vaidyanathan, a spokeswoman for the ACEEE.
When operating under gasoline power, the Volt gets EPA-rated fuel economy of 35 miles per gallon in the city and 40 on the highway.
That's not as good as the third-ranked Smart ForTwo, which gets 41 mpg on the highway, 33 in the city or the fourth-ranked Toyota Prius, which gets 48 on the highway and 51 in the city.
A car's weight is considered an important factor into the rating system, so the Volt's nearly 3,800 pounds dragged its rankings down. The Chevrolet Cruze Eco, a gasoline-powered car that's otherwise very similar to the Volt but weighs 750 pounds less, ranked 8th on the list. Most of the Volt's additional weight comes from its lithium-ion battery pack.
For electric and hybrid cars the organization accounted for battery weight separately. Nickel-metal batteries used in hybrid cars like the Prius have a greater impact, pound for pound, than the less toxic, but far larger, lithium-ion batteries used in the Leaf and Volt.
Weight accounted for roughly 40 pct of the Volt's overall score.
All told, the Volt ended up ranking lower than seven non-hybrid gasoline powered cars.
"I find it kind of laughable," said GM spokesman Rob Peterson when told of ranking and the rationale behind it.
Peterson objected to the idea that the Volt's weight counted so heavily against the Volt.
"It's one group's interpretation of a measurement of 'green'," he said.
The ACEEE gave every 2011 model year a "Green Score" base on its overall environmental impact. Among the lowest-scoring vehicles of all were the Ford Expedition and the Chevrolet Suburban.
The ACEEE is a Washington-based non-profit group dedicated to "advancing energy efficiency as a means of promoting economic prosperity," according to its Web site. The group has produced an annual "Green Cars" guide since 1998.
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