Since the early 1990's, a large body of data has demonstrated that the properties of gasoline fuels have measurable effects on exhaust emissions from cars and trucks. Since that time, vehicle technologies have changed substantially and increasingly stringent emissions standards have been implemented, leading to marked reductions in exhaust emissions from motor vehicles. In model year 2004, cars and light trucks certified to Federal Tier 2 emissions standards entered the market. By 2017, we project that 70 percent of the car and light truck fleet will be comprised of Tier 2 vehicles, accounting for 80 percent of total vehicle miles travelled (VMT). Existing fuel-effects models, such as the EPA Predictive Model and the Complex Model1, were developed using data representing 1990s-technology vehicles meeting the Tier 0 and Tier 1 emission standards, levels an order of magnitude higher than current (Tier 2-compliant) vehicles2. With the fleet turning over to much lower-emitting vehicles, the Agency and stakeholders were interested in generating a coherent body of updated fuel-effects data, to provide the basis for generation of updated fuel effects models representing the gasoline vehicle fleet at the time of the study. In addition, in the Energy Policy Act of 2005 (EPAct), Congress required EPA to conduct the necessary research and develop updated models.