Vehicle Ratings Methodology
The WhatGreenCar vehicle rating system expresses a vehicle's lifecycle environmental impact as a score out of 100 ranging from
0 for the greenest vehicles to 100+ for the most polluting.
The database includes vehicle ratings for over 6000 models covering model years 2008 through 2010. Ratings are available for gasoline, diesel, bioethanol (flex-fuel) and natural gas light-duty vehicles that can be purchased in the US. During the fall of 2009, ratings for electric and fuel cell vehicles will also be available.
Vehicle ratings overview
The rating system used by WhatGreenCar is based on an assessment of the environmental impacts associated with a car's use and manufacture. This includes all aspects of producing and using the fuel - the fuel cycle (primary production, extraction, transportation, refining, and vehicle operation), as well as the vehicle's manufacture, assembly and disposal - the vehicle cycle.
The WhatGreenCar analysis first quantifies the extent of life cycle air emissions arising from the fuel and vehicle cycles (known as an emissions inventory). The air emissions assessed include the Certified emissions - carbon monoxide (CO), oxides of nitrogen (NOx), non-methane organic gases (NMOG) and particulates (PM) - and sulphur dioxide (SO2). In addition, the three main greenhouse gases associated with road transport are assessed: carbon dioxide (CO2) , nitrous oxide (N2O) and methane (CH4).
Next, the WhatGreenCar analysis conducts an emissions impact assessment - as its name suggests, this quantifies the impacts of the emissions rather than just quantifying the amount of emissions produced. The advantage of this approach is that the varying levels of all the emissions assessed can be combined to produce an overall environmental impact - without this approach it is difficult to know how to compare (for example) a car with high CO2 and low NOx, with a second vehicle that has low CO2 and high NOx.
The emissions impact assessment is achieved by the use of an environmental rating tool first developed by the European Cleaner Drive Programme to assess the impacts associated with the fuel cycle. This rating system uses recognised 'external costs' to establish the relative weight to attach to different emissions - the external costs are values expressed in monetary terms that reflect the overall damage to the environment and to human health. The analysis used by WhatGreenCar extends the Cleaner Drive method to include vehicle cycle (car manufacture and assembly).
Using the WhatGreenCar rating system, the level of environmental impacts are expressed as a score between 0-100 - the lower the score, the less the environmental impact (this reverses the Cleaner Drive scores which were higher for lower emission vehicles).
Input data for the WhatGreenCar rating methodology comes from number of reference sources including: the Environmental Protection Agency and California Air Resources Board (for vehicle or tailpipe emissions), the Argonne National Laboratory's GREET project for fuel production greenhouse gas emissions, and several academic papers that estimate the emissions produced during the production of materials used for vehicle manufacture. For a detailed report on the rating methodology used by WhatGreenCar, download the following pdf document.
Vehicle ratings in detail (pdf document)
For a detailed explanation of the WhatGreenCar Ratings methodology, download the pdf document: WhatGreenCar_US_Vehicle_Ratings_2009 (700kB)