Six of the Top 12 Greenest Cars

#1: Honda Civic GX

Mileage: 24 city, 36 hwy, per gallon-equivalent

Emissions are a major factor in the American Council for an Energy Efficient Economy's "Green Car" rankings, which the group has published annually since 1998. That's why the Civic GX tops the list despite fuel economy figures that are lower than the others. It runs on compressed natural, which is much cleaner burning than gasoline.

Emissions are why you won't find any diesel-powered cars on this list. Despite being very efficient, and much cleaner than they were in the past, diesel engines still emit more pollution than the most fuel-efficient gasoline cars.

#2: Nissan Leaf

Mileage: 106 city, 92 hwy, per gallon equivalent

EV rage: 100 miles

The Leaf burns no fuel at all, running purely on electricity. Electric power plants do burn fuel, and that's a factor taken into account in the ACEEE rankings. That's why the Leaf ranks second to the clean-burning Honda Civic GX.

#3: Smart ForTwo

Mileage: 33 city, 41 hwy

The ForTwo, a two-seat car, relies on small size, light weight and a tiny three cylinder engine to get a great "Green Score." Besides contributing the car's fuel efficiency, weight itself was a big factor in the ACEEE's scoring. The assumption is that heavier cars create more waste and emissions in their manufacturing and disposal than lighter cars.

#4: Toyota Prius

Mileage: 51 city, 48 hwy

Hybrid cars rank highly based on fuel economy, but they pay a penalty because of their heavy battery packs. Cars that use nickel-metal batteries, as the Prius does, pay a higher penalty in the rankings because their batteries contain more toxic substances than do the lithium-ion batteries used in most plug-in cars.

#5: Honda Civic Hybrid

Mileage: 40 city, 43 hwy

Because its hybrid system operates differently from that in a Prius, the Civic Hybrid is more efficient in highway driving than in the city. It's also less efficient overall.

#6: Honda Insight

Mileage: 40 city, 43 hwy

Honda launched the Insight as a less costly competitor to the Prius. Despite being a smaller car, its fuel economy is the same as the Civic Hybrid.