ACEEE publishes greenercars.org, an annual consumer-oriented guide offering environmental ratings for every new model in the US light-duty vehicle market. This report covers the data issues, key assumptions, and analytical methods used to develop the vehicle ratings for greenercars.org. Based on principles of life cycle assessment and environmental economics, the greenercars methodology estimates the impacts of criteria pollutant and greenhouse gas emissions throughout the vehicle life cycle and the full fuel cycle, including in-use emissions. The report summarizes the application of the methodology, highlights results for major classes and technology types, and identifies future research needs.