From Green Right Now Reports

How do cars pollute? In two main ways, through inefficient mileage (guzzling a gallon of gas every eight or 10 or 14 miles) and through tailpipe emissions.

There’s the pollution associated with manufacturing, also, but to keep it simple let’s stick with emissions and mileage. Obviously, both affect the air. Think of mileage as a measure of your car’s pollution volume over time – if a gallon of gas doesn’t take you very far, you have to burn a lot more gas — and emissions as the chemistry of that pollution; if the mix is particularly noxious, your car will be a bigger offender than one with better tailpipe controls.

So if you want to buy the cleanest car you can — in the price range you need — you’ll look at both factors. Fortunately, the U.S. Environmental Protection Agency has already done this work, assigning a  “greenhouse gas” score to most models. Find it at the EPA’s Green Vehicles website.

The EPA’s “greenhouse gas” score considers how much a vehicle contributes to global warming via its full lifecycle emissions of greenhouse gases (carbon dioxide (CO2), methane (CH4), nitrous oxide (NO2), and hydroflurocarbons (HFCs). The rating is mainly a function of a car’s gas mileage, but the analysis also looks at the fuel a car uses (gasoline, natural gas, diesel, ethanol) to factor in emissions.

Cars are given a score between 1 and 10, with 10 being the highest (the Prius solos at this rating) and 5 being not so hot (the Lincoln Town Car) and 3 (the GMC Sierra 15 gasoline model) being about as low as it goes. (The Sierra 15 jumps to a 6 on ethanol though.)

At the same site, the EPA chart also breaks out a column that considers just regulated tailpipe emissions — which doesn’t capture all the greenhouse gases and considers some outputs that aren’t greenhouse gases — giving each model an “air pollution score” as well. A diesel, say, might not score as well on this scale as it would on the greenhouse gas scale.

Interested in seeing just the EPA’s gas mileage ratings? Look to fueleconomy.gov, courtesy of the Department of Energy.