The Institute for Lifecycle Environmental Assessment, http://www.ilea.org, is an advocacy group in Seattle. You can follow its President, Roel Hammerschlag, as he reasons his way through the choices. Ranking them from 5 to 1, PHEVs are at the top -- though there is an "option 0".

A version of this article appeared in Solar Today, http://www.solartoday.org/­2005/­nov_dec05/­green_car.htm the magazine of the American Solar Energy Society

http://www.ilea.org/­articles/­car.html (also see matrix there) What Kind of Car?

You're concerned about the environment, but you need to drive. Which car will do the least damage? There's no easy answer, and you will have to make some tradeoffs between your budget and your determination to help change the world. But you will also need to think about what "changing the world" means to you:

Is it more important to (a) help reduce future emissions an uncertain amount by investing in advanced technology or (b) reduce immediate emissions a known amount with existing technology?

Is it more important to (a) fight climate change and foreign oil dependence by reducing fossil fuel use or (b) help clean the air in your region by reducing traditional pollutants from the car tailpipe?

At ILEA we are all about life-cycle assessment, so we like to take the big picture approach: advancing technology trumps personal emissions, and greenhouse gases trump local pollutants. If you answer the questions differently, keep those differences in mind as you read through our recommendations; near the end of the email there's a comparison table to help you do this.

Below are five basic choices you can take, beginning with the most conventional and ending with the most adventurous.[1] We think the most adventurous steps probably have the most impact on the big picture, but if you answered the questions differently than we do, you may want to take one of the other choices.

#5: High-efficiency conventional cars If none of the more advanced options meet your needs, then you can still reduce both your greenhouse gas and your traditional pollutant emissions by being careful to choose a car with the highest fuel economy. Keep in mind though, this option will do little to advance new technologies.

The U.S. EPA's fuel economy website provides comparative ratings of nearly all cars available in the U.S. The American Council for an Energy Efficient Economy publishes the Green Book, which provides even more detailed environmental information. Though the EPA website is free, the Green Book is not: a month of access costs $8.95.

Diesel engines are on average more efficient than gasoline engines. Just as one example, a manual transmission, 2005 VW Jetta Wagon is rated 36 city and 47 highway. In the United States we tend to think of diesel cars as dirty, and indeed historically the tailpipe emissions have been much worse than for gasoline cars. But beginning in mid-2006 all automotive diesel fuel in the U.S. will be ultra-low sulfur, allowing better emissions control and eliminating nearly all of the irritating exhaust fumes we normally expect from diesel engines. That means beginning in 2007, many diesel vehicles will be preferable to their gasoline counterparts. Also, if you choose a diesel vehicle, you will always be able to leapfrog to the much more aggressive biodiesel solution, #3 below.

#4: Gasoline-electric hybrids Gasoline-electric hybrids (usually just called "hybrids") have by far been the most popular choice of environmentally conscientious car buyers over the past few years. Hybrids are fueled at the gas pump just like any other gasoline-powered car, but boast particularly impressive mileage (the 2005 Toyota Prius does 60 city and 51 highway).[2]

A hybrid car has both a gasoline engine and an electric motor, but it does not need to be plugged in. The gasoline engine takes care of charging the electric motor's battery. The battery is also charged when the driver steps on the brakes: the electric motor works in reverse to stop the wheels by converting their rotational energy to electricity saved in the battery. Hybrids also cut out the gasoline engine when the car drives very slowly or stops, minimizing energy wasted in idling.

Gasoline-electric hybrids are an excellent choice for the environmentally aware consumer. Because of their high fuel economy, hybrids reduce both greenhouse gases and traditional pollutants, like any other high-mileage vehicle. But buying a hybrid gives the environment an extra boost because you are helping introduce an important, cutting-edge technology. So far, Honda and Toyota have released hybrid passenger cars, and Ford has released a hybrid SUV. You can expect to see many more models appear over the coming few years.

#3: Biodiesel You can nearly eliminate your greenhouse gas emissions by buying an ordinary, high-efficiency diesel vehicle and filling the tank with biodiesel. Biodiesel is made from recycled vegetable oils or from by-product oils of crops grown for other purposes. The net greenhouse gas emissions from making the biodiesel are extremely low, and when burned in a diesel engine biodiesel is cleaner at the tailpipe too.[3] As of early 2005, biodiesel sells for about $1 more per gallon in the U.S. (on average) than fossil diesel, but a new federal tax credit promises to lower the price down to nearly the same as fossil diesel.

With this choice you are boosting the toddler biodiesel fuel industry. Biodiesel will always be an environmental niche product, because there is not enough land area in the U.S. to grow crops for the entire vehicle fleet.[4] Still, it may be an important part of a future transportation solution. Because your car will always be able to burn fossil diesel fuel, it is important that you are committed to the effort to buy biodiesel, even if it means going out of your way each time you need to refill your tank. Before you make this choice, make absolutely sure that biodiesel is available in your area.[5] If it isn't and you are particularly excited about biodiesel, get together with some other enthusiasts and you can make your own.

#2: Electric cars A warning here. Once you step past option #3, biodiesel, you are stepping into the realm of the difficult, the expensive and oftentimes the nerdy. You may end up importing cars or car parts from other states or even countries, and engineering skill (or at least the skills necessary to talk to engineers) can get you more of what you want in a custom car. If you feel motivated for a technical adventure, read on.

Electric cars are falsely perceived as short in performance and short in range. In fact, electric cars can be stunning performers, as members of the National Electric Drag Racing Association will be all too happy to tell you (or show you). Though performance limitations are pure myth, range limitations are real. But recent advances in lithium-ion battery technology are bringing 300 mile ranges within reach.

Electricity is not always the best environmental choice. Though the car is zero-emissions on the road, the electric generator can sometimes be a terrible emitter of both greenhouse gases and traditional pollutants, especially if you live in a coal state. Fortunately, there is a solution. You can make sure to buy exclusively green electricity to charge your car; electricity is so much cheaper than gasoline that even the slightly more expensive green electricity will still come in well under the cost of gasoline.

Even without using green electricity, promoting electric cars has an enormous value for long-term energy policy. Electricity is by far the best way to get renewable resources to cars, and helping to get electric cars onto the market is an important enabling step for renewable energy in general. If you want to buy an electric car, start by visiting the Electric Drive Transportation Association website.

#1: Plug-in hybrids Driving a plug-in hybrid is probably the most edgy and policy-advancing choice you can make, but getting your hands on a plug-in hybrid will make buying an electric car look like a piece of cake: there are no commercial models available in the United States.

A plug-in hybrid is similar to a gasoline-electric hybrid, but it has a larger battery and can be plugged into a wall socket as well as fueled with gasoline. The plug-in hybrid can drive entirely on electric energy for twenty, thirty or forty miles depending on how large the battery is. Because most people use their cars for commutes of a few tens of miles at the most, a plug-in hybrid that is recharged each night can run almost exclusively on electricity, kicking in the gasoline engine only for longer trips.

Though you can't buy one, a sufficiently motivated enthusiast can convert a Toyota Prius to plug-in mode - at a steep cost. If you lack either the money or the technical savvy to do that, best just stay in tune with the evolving technology, and wait for your chance to buy one off the shelf.

Why is ILEA advocating a plug-in hybrid as the most edgy choice, when all-electric cars might seem more like the ultimate goal for transportation technology? Though engineers are well on their way to solving the all-electric range problem, they're not there yet. Plug-in hybrids are an extremely valuable bridge technology, because they retain the convenience of unlimited range that consumers are used to, while simultaneously advancing the large automotive batteries that can be the foundation for all-electric cars.

But maybe plug-in hybrids are in fact the best, ultimate goal for sustainable transportation. An advanced, plug-in hybrid with a 100 mile range would need to be fueled only a few times a year, in most cases. Such a low rate of liquid fuel consumption, if it pervades the car market, would enable us to fuel all of our vehicles with a renewable liquid fuel like ethanol or biodiesel, getting the best of both approaches.

#0: No car at all Even a zero-emissions car results in environmental impacts from generating the electricity or biofuel that powers it; upstream emissions from the car manufacturing plant; and indirect impacts from the roads and parking lots that support it. The best car is no car. If you can avoid purchasing a car by using one or more of the alternatives below, you will have the biggest personal impact on the environment possible:

• carpooling can easily triple your fuel economy on a per-passenger basis;
• using vanpools or public transit saves gas and saves you road-rage;
• car-share programs might give you just enough access to a vehicle that you don't have to buy one;
• combining your trips saves you time as well as gas;
• living in an urban neighborhood near your workplace, shops and other resources can eliminate most of your car trips;
• and of course, anytime you can walk or bicycle instead of drive, take the opportunity to get some healthy exercise!

[1] Some of you may notice the absence of "flexible-fuel" vehicles that can accept either gasoline or ethanol. ILEA is not promoting this solution because most ethanol available in the U.S. is starch-based, inefficiently using only a portion of the crop (usually corn) that generates the ethanol. Flexible-fuel vehicles will make more sense once cellulosic ethanol, which makes efficient use of the entire plant, becomes more widely available. Also, "flexible-fuel" vehicles are not a significant technological advance: their design is nearly identical to an ordinary, gasoline-fueled car.

[2] Hybrids have the curious property of achieving better mileage in-city than on the highway. The hybrid technology can take great advantage of the slow, stop-and-start driving, but is incapable of mitigating the wind resistance at highway speed.

[3] Office of Transportation and Air Quality. A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions. Washington, DC. U.S. EPA. 2002. EPA420-P-02-001.

[4] Charles L. Peterson. Potential Production of Biodiesel. University of Idaho.

[5] In some areas you may be limited to blends of biodiesel with fossil diesel. Blends are labeled with "B" numbers, for instance B20 is a blend of 20% biodiesel with 80% fossil diesel, B100 is 100% biodiesel (pure or "neat" biodiesel). In cold climates a blend may be necessary, because without additives B100 begins to gel around 32 degrees F, while B20 won't begin to gel until as low as 7 degrees F.