How is the Green Car Rating calculated?

The Green Car Rating 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 Green Car Rating 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 so-called 'regulated 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).

The analysis then 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 Next Green Car extends the Cleaner Drive method to include vehicle cycle (car manufacture and assembly).

Using the Green Car 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 Green Car Rating methodology comes from number of reference sources including: the Vehicle Certification Agency (for vehicle or tailpipe emissions), and several academic papers that estimate the emissions produced during the production of materials used for vehicle manufacture.