To quote a famous frog, "It ain't easy being green." As an environmentally conscious car shopper, you might add, "It ain't easy buying green, either." Sure, you can turn to car reviews, buying guides and Web sites that compare the price, horsepower, 0-to-60 time, head room and trunk space of various makes and models. Just try to compare environmental friendliness, though, and you'll end up echoing Kermit's lament.

With a little looking, however, you can find sources that rate the relative "greenness" of a vehicle. But they usually employ different standards for environmental friendliness--and sometimes come up with different results. Green, it seems, means different things to different people. A look behind vehicles' environmental ratings raises some fundamental questions. How can you distinguish between various shades of green? What does the phrase "environmentally friendly" really mean?

More Than mpg

For most people, fuel economy represents the ultimate measure of a vehicle's environmental impact. The higher the mpg, the greener the car--or so they think. In fact, fuel economy is only part of the story. "Typically, a vehicle that burns less fuel also pollutes less," says James Kliesch, research associate at the American Council for an Energy Efficient Economy (ACEEE), a Washington, D.C.-based research and advocacy group. "There's not necessarily a correlation, though. How a car is engineered can make all the difference in whether a fuel-efficient vehicle is clean or not so clean."

In fact, two vehicles can burn exactly the same amount of fuel per mile, but one will pollute less than the other. Two factors affect the outcome. First, some engines actually lower emissions when gasoline burns in the combustion chambers. For example, engineers may design an engine to run "lean"--that is, with relatively less fuel in the fuel-air mixture in the cylinders. "That's a great way to conserve fuel," notes Kliesch, "but it's also a great way to increase emissions." However, variable valve timing and other sophisticated technology can enable a leaner-running engine to minimize the pollution it would otherwise produce.

Then there's the so-called after-burn treatment--most notably, the catalytic converter or converters. (Some cars now have more than one.) Think of the catalytic converter as a little washing machine inside your car. Dirty exhaust goes in, the converter neutralizes much of the really bad stuff in it, and it comes out less dirty. Here again, differences in technology can lead to differences in cleanliness--all from the same amount of gasoline.

Not that fuel economy doesn't matter. For one thing, better overall fuel economy means less dependence on oil, with all of its environmental and geopolitical implications. For another, it means lower production of greenhouse gases. The stuff that comes out of your tailpipe after it passes through the "washing machine" provides a good illustration. Through chemical reactions, the catalytic converter "washes" much of the carbon monoxide (a colorless, odorless poison), changing it into carbon dioxide. It also changes at least some of the nitrogen oxides (NO and NO2, smog-forming lung irritants) into nitrous oxide (N2O), which you know better as laughing gas. Although carbon dioxide and nitrous oxide aren't dirty in the strictest sense, they're not exactly harmless, either. Both products contribute to global warming. The less of them we produce by burning gasoline, the better.

What the Feds Say

Essentially, a true measure of greenness must account for not only the quantity of fuel burned per mile, but also the quality of emissions produced. The U.S. Environmental Protection Agency provides a measure of both in its Green Vehicle Guide ( www.epa.gov/greenvehicles).

In its guide, EPA assigns a green score to each vehicle, from 0 to 10 (10 being the cleanest). The agency bases its scores primarily on two federally regulated pollutants--nitrogen oxides and hydrocarbons, both of which contribute to smog. Two other pollutants--particulate matter (fine airborne soot that can cause respiratory problems and perhaps even cancer) and carbon monoxide--also influence the scores, but to a lesser extent. Under the EPA's scoring system, the higher the score, the cleaner the emissions, as measured in grams of those four pollutants produced per mile. This year, more than a dozen of '04 models rate a perfect "10" on EPA's system.

But what about greenhouse gases and other, non-regulated pollutants? For those, you have to turn to the fuel economy ratings, which EPA provides right alongside the green score for each vehicle. When you shop for a vehicle, the best fuel economy combined with the highest green score indicates your greenest choice overall.

ACEEE's Green Book

EPA's Green Vehicle Guide makes no attempt to assign relative importance to either fuel economy or cleanliness in the overall environmental equation. For example, you might consider two vehicles--one with higher fuel economy but a lower green score than the other. Which is the environmentally friendlier choice overall? If you rely only on EPA's guide, you're on your own in answering that question.

One source--ACEEE's Green Book--comes to the rescue by "doing the math" for you. Available in bookstores or online ( www.greenercars.com) for a fee, it attempts to evaluate a vehicle's environmental impact more comprehensively by accounting for not only greenhouse gases and other non-regulated pollutants (as indicated by fuel economy) but also cleanliness (as measured by emissions standards). And it adds another element to the environmental equation--namely, pollution generated at the factory in manufacturing the vehicle. ACEEE uses the vehicle's weight to estimate its manufacturing impact. The heavier the vehicle, presumably the greater its environmental impact during production, disposal and scrap.

ACEEE then estimates the damage cost of each pollutant, in cents per mile. In theory, those damage costs reflect the harm to society in sickness, premature death, and other costs of pollution. In ACEEE's methodology, greenhouse gases account for half of the total environmental harm, and conventional pollutants account for the other half. Finally, ACEEE converts those costs to an overall environmental damage index for each vehicle. That index is reflected on a "green scale" running from 0 to 100. The higher the score, the greener the vehicle.

Comparing EPA's and ACEEE's scores for the same vehicle reveals how different methodologies yield much different results. For example, one version of the Subaru Legacy Wagon (2.5-liter, four-speed automatic with four-wheel drive, getting 22/28 mpg, city/highway) earns a perfect 10 on EPA's green scale. The same version of the same vehicle earns a green score of 33 from ACEEE--an "average" rating for its class and only a few points above the average of 27 for all vehicles covered in the guide.

Seeing Green in the Showroom

You can also get a general idea of a car's greenness in a showroom or new-car lot, without resorting to EPA or ACEEE ratings. First, look for the best fuel economy rating you can, as indicated by the EPA sticker on the window. Then, determine the emissions certification for that particular version. Lift the hood and look for the "Vehicle Emissions Control Information" label on its underside. Federal law requires this label, since different versions of the same make and model may have different engines, transmissions and emissions control equipment.

In addition to listing the engine size (in liters) and a 12-character identification number, the label will also specify the state or federal standards to which the vehicle is certified. New York and several other states have adopted updated standards pioneered by California, known as LEV II for low-emissions vehicles, phase II. The vehicle that meets the strictest LEV II standard becomes your "greenest" choice. (See "Clean, Cleaner, Cleanest" below for the certification hierarchy.)

In the end, environmental friendliness means a lot more than mere gas mileage. As Kermit would tell you, cars--like frogs--come in subtler shades of green, and it takes a keen eye to distinguish a particular hue.