How We Rate the Vehicles
We analyze automakers' test results for fuel economy and emissions as reported to the U.S. Environmental Protection Agency and the California Air Resources Board, along with other specifications reported by automakers. We estimate pollution from vehicle manufacturing, from the production and distribution of fuel and from vehicle tailpipes. We count air pollution, such as fine particles, nitrogen oxides, hydrocarbons and other pollutants according to the health problems caused by each pollutant. We then factor in greenhouse gases (such as carbon dioxide) and combine the emissions estimates into a Green Score that runs on a scale from 0 to 100. The top vehicle this year scores a 70, the average is 42 and the worst gas-guzzlers score a 19.
We have made some changes to our methodology for 2018. An account of those changes can be found here. Our complete methodology report can be found here.
The ACEEE's Greenercars.org Methodology
Many factors determine the environmental impact of a car or light truck. Tailpipe emissions and fuel efficiency are clearly important, but impacts also depend on the type of fuel used and the materials that go into manufacturing the vehicle. A scientific approach for estimating the environmental impacts of a product is known as lifecycle assessment, since it traces the impacts of a product from "cradle to grave": materials production and product manufacturing; emissions and other effects when the product is in use; through end-of-life effects of disposal and recycling. We developed the green scores and class rankings according to the principles of lifecycle assessment, using available data that are sufficiently standardized to be applicable to all makes and models.
Four types of vehicle-specific data form the basis of the ACEEE's ratings: tailpipe emissions, given by the emissions standard to which a vehicle is certified; fuel economy, based on EPA test cycles; vehicle mass (curb weight); and battery mass and composition (for hybrids and plug-in vehicles).
In real-world driving, tailpipe pollution (CO, HC, NOx, and PM) can be substantially higher than the nominal grams-per-mile (g/mi) emission standard to which a vehicle is certified. These excess emissions occur for a variety of reasons: inaccuracy of the tests; malfunctioning emission control systems; and deterioration of the catalytic converter and other components. We previously applied adjustment factors, similar to those used in EPA's MOBILE model, to determine the expected lifetime average emissions for vehicles meeting a given standard. With the completion of the phase-in of the tier 2 tailpipe pollution control program, the earlier adjustment factors are outdated, so we currently assume vehicle emissions at the level of the standards to which they are certified. In the past we used 50,000 mile standards but we have since switched to using full useful life standards to rate vehicles (120,000 miles).
Fuel economy data are used to calculate greenhouse gas emissions, fuel-cycle criteria emissions (air pollution due to producing and distributing the fuel), and those aspects of vehicle emissions that are related to fuel consumption rates. Fuel economy determines a vehicle's energy consumption rate (gallons/mile, or kWh/mile or Btu/mile for electric and alternative-fuel vehicles). This value is multiplied by national average emission factors for the various pollutants to give emission rates in grams per mile.
Vehicle and battery weights are used as the basis for estimating manufacturing and disposal impacts. Standardized, model-specific data on the environmental damage of vehicle manufacturing are not available. With this year’s methodology change, we draw from the vehicle life-cycle module of Argonne National Laboratory’s GREET model to generate weight-based estimates of impacts that vary by technology. For hybrid and electric vehicles, GREET accounts for the replacement batteries needed over the vehicle's lifetime.
Having determined the average emission rates for each major stage of the vehicle's lifecycle (including those associated with the fuel consumed), the next step is to determine the relative environmental damage done by each pollutant. An economics-based approach for assessing environmental harm involves estimates of damage costs associated with a given pollutant. Specified, for example, in cents per gram (¢/g) of pollutant, these estimates reflect the costs to society of illnesses and premature deaths associated with pollution. Damage cost estimation involves uncertainties, of course, but it may also fail to reflect the full value we place on our health, environmental quality, and the protection of ecosystems. In spite of these limitations, damage costs provide a rational and consistent way to account for the different effects of various pollutants, and so we apply them to the emissions rates calculated from the vehicle data.
It is very difficult to estimate a damage cost for CO2 and other greenhouse gases. The damage due to global warming is just beginning to occur and the worst risks are largely in the future. Therefore, we cannot look back at the harm that has already occurred—as has been done for conventionally regulated pollutants such as NOx and PM—in order to estimate damage costs. However, because of the grave risks and growing concerns about greenhouse gas emissions, green book methodology initially gave global warming concerns equal weight to other forms of air pollution in determining our green vehicle ratings. Therefore, we assigned CO2 emissions a cost value such that, for the average model year 1998 light duty vehicle, approximately half of the overall environmental harm was associated with global warming risks and the other half is associated with the health effects of conventional air pollutants. We have kept the CO2 emissions damage cost constant over time. Consequently large gains in tailpipe pollution control and relatively minor fuel economy gains yield a roughly 70-30 split of damages between greenhouse gases and conventional pollutants on average for model year 2018.
Multiplying the gram-per-mile pollutant rates by their appropriate cents-per-gram damage costs (which vary by pollutant and location of emissions) yields environmental impact estimates in cents per mile (¢/mi). Adding up the ¢/mi estimates for all pollutants, including greenhouse gases, gives a total impact estimate for a given vehicle, which we term its environmental damage index (EDX). The EDX is the main result of our analysis for each vehicle and it provides the common metric with which we compare different makes and models. The EDX represents environmental harm; thus, the lower the EDX, the greener the vehicle.
For a green scoring system, greener vehicles should get higher scores. Therefore, we converted the EDX to a Green Score on a scale of 0-100 by grading along a curve, using a formula specified so that an EDX of zero corresponds to a Green Score of 100.
Finally, to determine the class ranking symbols, we examined the range of EDX values within each vehicle class. Cut points were determined on the basis of the distribution unique to each class. In addition, for a model to earn a "superior" class rating, its Green Score must be better than the overall average Green Score, as well as being among the highest in its class. This year, the overall average EDX is 1.58¢/mi, corresponding to a green score of 42.