Tip: How to Buy 'Green' Cars

With fuel economy on every car buyer's mind, the transition to more environmentally conscious vehicles is becoming a reality. However, while fuel economy is a fundamental part of the 'green' car equation, it isn't the only measure of the impact our transportation choices have on the environment.

Beyond shopping fuel economy, finding a car with superior green credentials has almost always been a challenge. While the Environmental Protection Agency (EPA) does classify tailpipe emissions by 'Tier 2 Bin' standards as a measure of environmental impact, these designations are meaningless to the average car buyer. Those standards are further confused by the alphabet-soup array of emissions classifications from the California Air Resources Board (CARB) that are also far from consumer friendly. SULEV, for example, is short for 'Super Ultra Low Emissions Vehicle,' an impressive sounding rating ... but what does it really mean?

To help shoppers better navigate the green car field, EPA offers a program called SmartWay and an online Green Vehicle Guide at www.epa.gov/greenvehicles. To earn SmartWay certification, a vehicle must have above average fuel economy, low tailpipe emissions, and low CO2 emissions per mile.

EPA rates vehicles from 0-10 points, with 10 being best for air pollution and Greenhouse Gas (CO2) emissions. A 5 point rating is average. To achieve the SmartWay rating, a vehicle must score 6 or better in the air pollution and CO2 categories and feature a combined score of at least 13 points. This doesn't necessarily mean that one of the two categories must be a 7, because a half-point is possible and two 6.5s could also make the necessary 13 points. Vehicle scores and SmartWay status are relative to other vehicles and model years with the same scoring thresholds so comparisons can be made. Adjustments are made to compensate for changes in the stricter 2008 EPA fuel economy testing.

Those who want a more technical look at how this all comes together can get their fix here, otherwise you can skip a paragraph ahead: Air Pollution (tailpipe) emissions include testing for NMOG (non-methane organic gases), NMHC (non-methane hydrocarbons), THC (total hydrocarbons), NOx (oxides of nitrogen), PM (particulate matter), CO (carbon monoxide), and formaldehyde. A score of 10 in the air pollution score would require achieving a CARB ZEV (Zero Emissions Vehicle) equivalent or EPA Tier 2 Bin 1 standard. A 9 in the air pollution category relates to a SULEV II score from CARB or Tier 2 Bin 2 from EPA.

SmartWay marks the first time that EPA has included carbon dioxide in an emissions rating. Since CO2 can predictably be estimated by the amount of a given fuel burned, EPA's Greenhouse Gas score has a direct correlation to a vehicle's fuel economy. CO2 emissions are calculated by factoring the combined highway and city fuel economy rating by a known number for the fuel burned. Fuels include gasoline, diesel, E85 ethanol, LPG (liquid petroleum gas, or propane), and CNG (compressed natural gas). Typically, gasoline-electric hybrids and CNG fueled cars like the Honda Civic GX deliver the lowest CO2 emissions and score a 9 or better in the Greenhouse Gas category.

Combined fuel economy is primary to the Greenhouse Gas score. That number isn't simply an average of the EPA city and highway fuel economy figures, however. EPA assumes that you drive a bit more in the city than on the highway, so the combined number is weighted at 55 percent city driving and 45 percent highway driving.

For the 2008 model year the EPA rated 2,648 models and configurations of cars and light trucks. Of these, just 26 percent - or 692 vehicles - make the cut for a SmartWay designation. Greener yet is the SmartWay Elite rating and only a handful of vehicles earn this distinction. Among the year's SmartWay Elite vehicles are the Ford Escape Hybrid, Honda Civic GX, Honda Civic Hybrid, Mazda Tribute Hybrid, Mercury Mariner Hybrid, Nissan Altima Hybrid, Toyota Camry Hybrid, and Toyota Prius.

The Green Vehicle Guide page on EPA's website allows searching vehicle scores by individual vehicle and vehicle type, and also calculates the greenest vehicles among those available in a given state. This function of segmenting by state is necessary since some follow stricter California standards so engine, transmission, and even model choices available in some states may not be available everywhere.

EPA's efforts in making emissions ratings easier to understand are a real plus for car buyers today. Combining traditional emissions ratings with those for greenhouse gases adds an important extra element that's especially meaningful today, given the huge interest in GHG emissions and fuel economy. Making environmental comparisons easy - and thus empowering consumers to buy 'greener' - is an important step forward in the successful implementation of green vehicles on a mass scale.