What makes one car greener than another?

That’s a good question, and a complex one.

How about organic hemp seat covers? Recycled pop bottles in the bumpers? Plastics made from soy beans?

All of these ideas hold promise for reducing the environmental impact of cars and automakers have been working some of them into their designs. For example, Ford Motor Company is already using soy-based polyurethane foam for seat backs and cushions in several vehicles, including the Focus, Escape, Mustang, Expedition SUV and F-150 pickups.

What about alternative fuel vehicles (AFVs)? They are promoted by car companies, government agencies and numerous interest groups as the way to both “get off of oil” and save the planet. AFVs run on something other than petroleum-based gasoline and diesel. Being non-petroleum by definition, of course they reduce oil use. But the environmental benefit of AFVs turns out to be a mixed bag. Some are better, some are not, and it’s a rare AFV that beats the cleanest gasoline cars on the market.

The most advanced gasoline cars such as the Toyota Prius are hybrid electric vehicles, which are often lumped together with alternatives. However, such “grid-free” hybrids run completely on gasoline and derive their green points from a combination of high fuel economy and low emissions rather than anything having to do with an alternative fuel.

Sources of pollution for a typical late-model car

It’s that combo of high fuel economy and tight tailpipe control that is in fact most important for making one car greener than another. Although the materials of which a car is made matter, the pollution from fuel use when a car is driven matters much more for its total environmental impact.

The adjoining chart shows the breakdown of the total environmental impact for a typical late-model light duty vehicle in the United States.

The materials that go into a car and emission during vehicle assembly amount to about 10% of the total on average. So while green materials are helpful, especially considering the huge number of vehicles produced, changing the material doesn’t have a big effect on how most vehicles score. And as a practical matter when comparing cars, there’s no way to trace where the materials came from, how they were made, or what portion of them might have been recycled or bio-based. Lacking such data, AutoEcoRating scores material impacts in proportion to how much a car weighs.

Most pollution now comes from the tailpipe and the next largest portion is from extracting and refining crude oil (including air pollution, greenhouse gas emissions and oil spills). For gasoline, the global warming portion of the emissions amount to 19 pounds CO2-equivalent per gallon plus 6 more pounds from extracting and refining the fuel, or a total of 25 pounds of CO2-equivalent greenhouse gas emissions per gallon of gasoline burned. Together, these fuel-related emissions amount to 90% of a typical car’s environmental impact. Thanks to the U.S. Environmental Protection Agency, we do have good data on fuel economy and tailpipe emissions, and so we can accurately compare cars on the basis of the factors that matter the most.

Of course, an electric vehicle has no tailpipe, but in that case the vast majority of the impact is associated with electricity generation. That includes both pollution from power plants themselves as well as from supplying them with fossil fuels, such as pollution from coal mining and natural gas extraction, and also radiation risks for nuclear power. So the impact of an EV is also largely related to how much energy it consumes. With their heavier batteries and lack of tailpipe pollution, the materials and manufacturing impacts end up being a larger share of the total. Nevertheless, a small EV easily tops the AutoEcoRating scale due to its combination of very low energy consumption and absence of tailpipe pollution.

In any case, no matter what the source of fuel, the environmental impact is lower if you use less of it. All of the fuels widely available today are still produced in ways that emit a lot of global warming pollution, and though some alternatives may be a bit better than others, there’s no clear winner overall. This is even more true once you factor in the economics, because alternatives are more costly.

The good news is that a car is greener if it gets better gas mileage regardless of the fuel and greener still if it has very low tailpipe emissions. That means that there are a lot of choices for consumers who want to comparison shop with the environment in mind. It just comes down to selecting the cleanest and most efficient vehicle that meets your needs and fits your budget. By combining clean and efficient into a single score, AutoEcoRating makes it easier to make a greener choice.


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