Cars are complicated and figuring out if our cars are hurting the environment is not as straightforward as "how many miles per gallon do we get?". One non-profit that is widely recognized as the leader in comparing the environmental impact of cars and trucks is ACEEE. 

ACEEE publishesACEEE's Green Book^®: The Environmental Guide to Cars and Trucks, an annual consumer-oriented guide providing environmental rating information for every new model in the U.S. light-duty vehicle market.

According to ACEEE.org, "the environmental rating methodology for ACEEE's Green Book^® is based on principles of life cycle assessment and environmental economics. The method is designed to be applicable given the limitations of data available by make and model in the U.S. market. The approach combines the impacts of traditionally regulated (criteria) pollutants with those of greenhouse gas emissions, covering both the vehicle life cycle and the fuel cycle, using a mass-based characterization of vehicle manufacturing impacts. This report covers the data issues, key assumptions, and analysis methods used to develop ACEEE's vehicle ratings. It summarizes the application of the methodology to the 2004 model year (MY), highlighting results for major classes and technology types, and identifies research needs for updating and refining the methodology. Appendices detail the parameters used to evaluate vehicles and document updates to the methodology since it original release in 1998."