Environment scores are assigned to cars by combining product-level environmental indicators (weighted at 75%) with GoodGuide's standard company indicators of environmental performance (weighted at 25%).
Product-level scores for this category are based on the following indicators:
- Fuel economy, as measured by EPA estimated miles per gallon for combined city and highway driving
- Greenhouse gas impact, as measured by scores developed by the US Environmental Protection Agency (EPA) and the California Air Resources Board (CARB)
- Local air pollution impact (e.g., smog); as measured by scores developed by EPA and CARB
- Non-driving impacts, covering the environmental damage created by the production and disposal of a car (i.e., materials extraction and processing, auto manufacturing, and end-of-life disposal)
Health scores are not assigned to cars. Instead, GoodGuide reports a car's safety rating from the National Highway Traffic Safety Administration (from one to five stars). NHTSA safety ratings are based on averaged crash test results for front, side, and rollover crash tests. GoodGuide also displays whether Electronic Stability Control is available for that car.
Social scores are based on company-level research. Product-level data on social performance are generally unavailable for this category of products.
Life cycle impact studies consistently conclude that the use phase of cars–when they are driven–is responsible for the largest proportion of their total environmental impact. GoodGuide's scoring system therefore gives the largest weight to driving impacts, as measured by a car's fuel economy (40%), greenhouse gas emissions (20%) and local air pollutant emissions (20%). Non-driving impacts associated with raw materials extraction, auto manufacturing, and end-of-life disposal are weighted at 20%.
For each indicator selected to characterize the environmental impacts of cars, we applied the following scoring rules:
- Fuel Economy — Vehicles are rated based on their estimated miles per gallon (MPG), which is calculated by combining their MPG values for city and highway driving using EPA methodology. GoodGuide utilized the 0-8 range of its rating scale to distinguish between most cars on the market - vehicles with combined MPG performance between 9 and 31 MPG are scored linearly within this range. GoodGuide reserved the 8-10 range of its rating scale for the most efficient cars on the market (defined as the top 5% of cars based on MPG ). Cars between 32 and 50 MPG are assigned scores linearly distributed between 8 and 9, while cars with MPG values of 50+ are awarded a 10.
- Greenhouse Gas Impact — GoodGuide uses Greenhouse Gas Scores developed by EPA and the California Air Resources Board to evaluate a car's potential contribution to climate change. These regulatory agencies use a 0-10 scale to score a car based on its emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), which are largely dependent on a vehicle's fuel type and the fuel economy of the engine. The California scoring methodology is more stringent than the EPA methodology because CARB incorporates the energy needed for fuel production into its scores, whereas EPA only takes into account tailpipe emissions. As a result, the emission thresholds used by CARB to establish scoring bins are slightly higher than those of EPA. GoodGuide did not attempt to adjust EPA or CARB scores to address this difference and reports EPA or CARB 0-10 scores without alteration.
- Local Air Pollution (Smog) Impact — GoodGuide uses Air Pollution Scores developed by EPA and the California Air Resources Board to evaluate a car's emissions of criteria air pollutants that contribute to local and regional smog problems. These regulatory agencies use a 0-10 scale to score a car based on tailpipe emissions of nitrogen oxides, hydrocarbons, particulate matter and carbon monoxide. GoodGuide reports EPA or CARB 0-10 scores without alteration. Note that otherwise identical cars that had conflicting air pollution scores in the EPA database were assigned the conservative (lower) score.
- Non-driving Impacts — The production and disposal of a car has a different pattern of impacts than its use phase, which is driven by fuel consumption. While different types of cars (electric, hybrid, gas or diesel) share many common vehicle components (e.g., car bodies), there is one key component that differs – the battery. LCA studies have documented that vehicle types with larger batteries, such as electric and plug-in electric cars, contain more embodied energy than those with smaller batteries, such as hybrid-electric vehicles. Internal combustion vehicles that run on gasoline, diesel, or natural gas do not use a separate battery for propulsion and therefore do not require this extra energy in their supply chain. GoodGuide therefore scores conventional combustion vehicles highest on this indicator, followed by hybrids, with all-electric vehicles receiving the lowest score.
GoodGuide's car ratings list vehicles that are made for the California (“CA”) market separately from similar models made for the rest of the US market. California has long taken the lead in controlling vehicle pollution, and cars sold in California typically have more stringent emissions controls than those sold elsewhere. Note that the The “CA” sales region also includes 15 additional states that have adopted California emission standards. “US” sales region includes states that are subject to EPA emission standards.
In order to distinguish between variations of the same model, car names generally conform to the following convention:
[Year] [Make] [Model] [Engine Displacement] [Transmission] [2 or 4 Wheel Drive] [Fuel] [Sales Region]
There have been significant changes in how regulatory agencies estimate Fuel Economy and Safety indicators that limit comparisons across model years.
EPA changed how it estimates MPG starting in model year 2008 by adding in the effects of faster speeds and acceleration, air conditioner use and cold weather. As a result, most vehicles receive a lower MPG estimate than under earlier methodologies.
NHTSA changed how it assigns safety ratings in model year 2011 by incorporating tougher tests that provide more information about vehicle safety and crash avoidance technologies. Ratings for 2011 and newer vehicles should not be compared to NHTSA ratings for 1990-2010 models.