There are actually two scores that the EPA gives to vehicles when putting together its Green Vehicle Guide. First there's the air pollution score, which measures how substantially a car's tailpipe emissions affects local and regional air quality. Any issues such as smog, haze and other problems caused by carbon emissions are all a part of air pollution. Vehicles are scored on a scale of 1 to 10, with 1 being dirtiest and 10 being the cleanest, so it's accessible and easy to understand which cars are best fit for eco-friendly driving.
Whereas the air pollution score takes into account the way your exhaust will affect nearby areas, the greenhouse gas score has a slightly broader scope in mind. It also takes into account a vehicle's fuel efficiency, or essentially an estimate of how many miles per gallon a car can get during a trip in the city or down the highway. These numbers are readily available; you can find them listed on the window stickers of new cars, inside the Fuel Economy Guide published each year by the EPA and the Department of Energy, or at auto dealerships, public libraries and Web sites like fueleconomy.gov.
The reason for establishing a greenhouse gas score is to take note of vehicles that can travel the same distance as another car yet burn less fuel in the process. When fuel consumption is lower, the amount of greenhouse gases emitted into the atmosphere -- mainly carbon dioxide, nitrous oxide and methane -- is also lower. Just like the air pollution score, the greenhouse gas score is based on a scale of 1 to 10, and the higher your car scores, the less your car contributes to global warming and climate change.
Carbon emissions from various fuels will vary because every fuel type burns differently. The amount of carbon released from gasoline in one mile, for instance, is not the same as the amount released from diesel or clean natural gas. The scores take into account not just petroleum-based fuels like gasoline and diesel but renewable fuels like E85. While the EPA considers the whole lifecycle of fuels, including the extraction, refining and distribution of products, it doesn't count vehicle manufacturing into its score. That's one more area you'll need to investigate on your own if you want a truly green car.
For lots more information on cars, vehicle emissions standards and green driving, follow the links on the next page.