What makes a car green? It's more than just its paint job. Choosing a green car is a more complex process than many realize. A hybrid badge slapped on a massive SUV does not automatically make the car planet-friendly. At the same, some sedans expel pollutants at a rate that would make a truck blush.

While it's a good place to start, gas mileage isn't the only issue to take into account when evaluating the effects of a car on the environment. Let's take a look at some of the main factors to consider when classifying a car as green.

Emissions

When looking at the direct environmental impact of a vehicle, it's most logical to begin with its emissions. While cars produce a number of pollutants, the one that is generally focused on is carbon dioxide (CO2) since it's a greenhouse gas and contributes to global warming.

An excellent place to find information on the CO2 emissions of a particular vehicle is www.fueleconomy.gov, a site maintained jointly between the Environmental Protection Agency and the Department of Energy. The EPA also has a handy Green Vehicle Guide where you can look up emissions statistics, as well as check on what vehicles in your state have earned the EPA's "SmartWay" and even more exclusive "SmartWay Elite" designations. These are given to cars with low emissions and high fuel economy that earn high marks in the EPA's Air Pollution and Greenhouse scoring categories.

One important note to consider is that electric cars produce no direct emissions. Some critics of electric cars note that they get power from electricity derived from coal. However, even with this logic, they still produce considerably less CO2 than standard vehicles. A chart at treehugger.com shows an emissions comparison between conventional, hybrid-electric and plug-in hybrid-electric vehicles. The emissions produced by electric cars depends greatly on their source of electricity.

Some also point to ethanol as a way to reduce CO2 emissions from cars. The benefits of ethanol are widely-debated. In some ways it works as a cycle: the crops that the ethanol is derived from soak up CO2 during their lifecycle. This CO2 is then dispersed back into the atmosphere when a car burns the ethanol fuel. Unfortunately, this view ignores the significant emissions released in the production of ethanol. Yahoo cites a study done at the University of California at Berkeley where researchers "concluded that using ethanol made from corn instead of gasoline would lead to a moderate 13 percent reduction in greenhouse emissions."

Fuel Economy

There's more to fuel economy than just the emissions. The Union of Concerned Scientists explains: "For every gallon of gasoline that is consumed, approximately 24 pounds of global warming pollution are released into the air. Drilling, refining, and distributing gasoline account for about 5 pounds of global warming pollution per gallon of gasoline, and burning gasoline during vehicle operation produces another 19 pounds of global warming pollution per gallon." That being said, better MPG translates to less pollution -- so small, light cars and hybrids tend to be good choices. The aforementioned www.fueleconomy.gov is the best source for mileage information on cars and trucks.

Green Manufacturing

Subaru of Indiana is widely regarded as the first zero-landfill plant, meaning it sends no waste to landfills during the manufacturing process. The waste is recycled, reused or converted to electricity. Zero-landfill plants have become increasingly popular, with other automakers such as Honda, Toyota and General Motors adopting the model.  Cars built at zero-landfill factories may not have the least emissions or use the least amount of gas, but they have the benefit of contributing less to landfill waste. 

Battery Disposal and Recycling

Some people are concerned about the nickel metal hydride batteries that are found in hybrids. More specifically, people worry about their proper recycling. The dangers associated with conventional lead batteries ending up in landfills are well-documented, but how do they compare to hybrids? HybridCars says, "While batteries like lead acid or nickel cadmium are incredibly bad for the environment, the toxicity levels and environmental impact of nickel metal hydride batteries -- the type currently used in hybrids -- are much lower."

Toyota goes the extra mile to assure that their batteries are properly recycled. They put a phone number and a $200 "bounty" on every one of their hybrid batteries.

The Brand Behind the Car

The manufacturers themselves are increasingly going green as well. They understand that consumers are intelligent and can distinguish between which green efforts are just for public relations and which ones actually produce benefits.

Toyota puts out an annual environmental report. Ford has made strides recently with its capless fuel filler system. In collaboration with Segway, General Motors has unveiled Project P.U.M.A. (Personal Urban Mobility & Accessibility), an energy-efficient prototype two-wheeler. And Honda has been making headway with efforts to reduce the transport costs of its cars.

What You Can Do

While there are many outside factors that contribute to a vehicle's greenness, some of the burden falls on you as the driver. Keep up on your car's maintenance with regular air filter and oil changes to boost your MPG significantly. Drive less, and when you do drive, maximize fuel economy. 

Between finding the right green car and using your vehicle responsibly, you can help minimize your impact on the environment.