ACEEE Updates Green Book® Online Methodology as U.S. Vehicle Market Diversifies

Blog Post | September 22, 2011 - 12:44 pm
By Shruti Vaidyanathan, Senior Advisor for Research

The uncertain economy and fluctuating gasoline prices have made consumers more discerning about their vehicle purchases. At the same time, the federal government is taking steps to ensure that future generations of vehicles will use less petroleum and emit less greenhouse gases.  In July 2011, the U.S. Environmental Protection Agency and the Department of Transportation presented a plan to strengthen fuel economy and greenhouse gas standards for cars and light trucks in 2017–2025, including raising  fuel economy in 2025 to 75 percent above 2010 levels—the largest step taken to improve fuel economy standards in the last 35 years.

Manufacturers are responding to these circumstances with an ever-increasing array of vehicle sizes and technologies. Model year 2011 saw the arrival of the country’s first plug-in hybrid electric vehicle (the Chevrolet Volt) and mass-produced battery-electric vehicle (the Nissan Leaf), along with a wide range of more fuel-efficient gasoline- and diesel-powered vehicles. The U.S. vehicle market is rapidly evolving into a highly competitive showroom of diverse vehicle types, sizes, and technologies for customers with different needs.

In this setting, we have updated the vehicle rating methodology for ACEEE’s Green Book® Online ( The Green Book® serves as a valuable resource for buyers who want to consider the efficiency and environmental friendliness of their new vehicle purchases. The Green Book® evaluates vehicles on a lifecycle basis to estimate greenhouse gas and criteria pollution emissions from vehicle manufacturing, operation, and recycling, and disposal.

This year, the “Greenest” vehicles list was highly contested. Hotly anticipated arrivals to the U.S. market placed, but did not win, in our rankings. The Nissan Leaf, for instance, came in 2nd behind the Honda Civic GX. Hybrids were pushed down a few spots, and in some cases replaced, by other highly efficient gasoline vehicles.

Part of the shake-up was due to our methodology updates, which we undertook to address changes in emissions modeling and advances in vehicle technology. Among the changes are the way emission standards are used to rate vehicles and the way emissions from the vehicle manufacturing, assembly, and disposal processes are quantified.

For the first time, we differentiated between a pound of battery and a pound of body weight in estimating manufacturing, recycling, and disposal impacts, based on Argonne National Lab’s GREET model. This tended to slightly lower the Green Scores of hybrid-electric and battery-electric vehicles because per-pound energy use and emissions associated with battery manufacturing and disposal are relatively high. We also evaluated electric vehicles based on the average national electric grid mix, which leans heavily on coal. These vehicles’ Green Scores will rise as the grid becomes cleaner.

Additionally, for criteria pollutants, we scored vehicles based directly on the emission standards to which they are certified, rather than using a real-world adjustment as prior Green Book® editions have. 

For further information on ACEEE’s vehicle rankings, the updated methodology is documented in the report Rating the Environmental Impacts of Motor Vehicles: ACEEE’s Green Book® Methodology, 2011 Edition.

In addition to the “Greenest” list, ACEEE’s Green Book® Online also identifies the “Meanest” vehicles on the market as well as a selection of top, widely-available models in each vehicle class. The list demonstrates that consumers can make “Greener Choices” whatever their vehicle needs may be.