Our numerical “ecoratings” or green car scores (here dubbed “eRatings”) are given on a purely relative, “higher is better” scale that is in inverse proportion to a vehicle’s total environmental impact. That impact is calculated using AutoEcoRating’s proprietary algorithm, which includes both smog-forming air pollution and greenhouse gas (GHG) emissions from production and use of the vehicle and its fuel, as well as other significant environmental harms for which adequate data are available.
Inputs to the algorithm include fuel economy, the tailpipe emissions standard that a vehicle meets and curb weight. AutoEcoRating blends these vehicle-specific data with sets of U.S. national average emissions factors and weighting parameters for major pollutants that are in turn based on government reports regarding average emissions associated with the production and use of fuels and materials. Our FAQ on how we combine the vehicle data with the other parameters provides a deeper dive into the technicalities for those who are interested.
The key output of the AutoEcoRating algorithm is the total environmental impact given as tons of pollution averaged over a vehicle’s lifetime. This impact number uses metric tons of carbon dioxide (CO2) as a common denominator. It’s not the same, however, as a carbon footprint based on, say, “tons of CO2-equivalent,” as commonly seen in discussions of global warming. That’s because it incorporates many impacts beyond CO2 and is only using the tons of CO2 units to map the diverse impacts to a single dimension, enabling us to offer consumers a single number for comparing cars.
To obtain the eRating itself, we take the inverse of the calculated impact and multiply it by a constant to put it on a simple numerical scale. We selected the scale to spread out the numbers enough to easily distinguish closely competing vehicles without using decimal places. The choice of scale doesn’t affect comparisons between vehicles, and because the scale is inversely proportional to the amount of harm to the environment, a car having an eRating twice as high as another’s has half the environmental impact. The graph below shows the relation between the environmental impact and the corresponding eRating, and highlights the results for vehicles that span a range of impacts.