It has long been recognized that vehicles emit more pollutants than allowed under the new car emission standards. Further tightening of the certification standards based on existing test procedures does not directly address the largest sources of emissions. This study attempts to quantify vehicle emissions by source, in order to prioritize future policymaking. Several new sets of data are used in conjunction with regulatory emission models to characterize the lifetime emissions from the average Model Year (MY)93 vehicle. Special attention is paid to two of the largest sources of real-world emissions: (1) high-power driving by cars with properly functioning emissions controls, and (2) cars with malfunctioning emissions controls. Emissions are projected to MY2000 and 2010, based on estimates of the effectiveness of recently adopted and proposed regulatory policies. These new policies are projected to reduce total emissions substantially.