A key motivator for wider deployment of electric vehicles (EVs) vehicles that are fully powered by battery charged from grid electricity is to bring about environmental cleanliness. This goal is based on the fact that EVs produce zero tailpipe emissioon the associated carbon emissins. However, the generation and transmission of the charge electricity produce emissions that are not explicitly accounted by current measurement metrics for EV greenhouse gas (GHG) emissions and as such, the notion of environmental cleanliness of EVs becomes questionable. In this paper, we propose a comprehensive metric to quantify the actual environmental impacts of EVs. The new metric that we call the electric vehicle emissions index (EVEI) captures CO2 emissions in the electricity production to consumption stages. Our metric is the first that provides transparency in the comparison of total emissions among various EV models, as well as in the side-by-side comparison of an EV with a gasoline vehicle (GV). Illustrative results indicate that the actual environmental impacts of an EV may show wide spatial variations and in some case, these impacts may be even greater than that of GV. Such insights that the EVEI provides may be useful in a wide range of applications, particularly in policy and incentive formulation.