As part of an ongoing assessment of the potential for reducing greenhouse gas emissions of light-duty vehicles, the U.S. Environmental Protection Agency has implemented an updated methodology for applying the results of full vehicle simulations to the range of vehicles across the entire fleet. The key elements of the updated methodology explored for this paper, responsive to stakeholder input on the Agency’s fleet compliance modeling, include 1) greater transparency in the process used to determine technology effectiveness, and 2) a more direct incorporation of full vehicle simulation results.This paper begins with a summary of the methodology for representing existing technology implementations in the baseline fleet using EPA’s Advanced Light-duty Powertrain (ALPHA) full vehicle simulation. To characterize future technologies, a full factorial ALPHA simulation of every conventional technology combination to be considered was conducted. The vehicle simulation results were used to automatically generate response surface equations (RSEs), enabling the use of a quick and easily implemented set of specific equations to estimate fleet-wide emissions in place of running time consuming full vehicle simulations for each potential technology package applied to each model in the fleet. Since the regressions were not extended to represent technology combinations that were not actually simulated, the emissions estimates produced from the RSEs match the ALPHA simulation results with a high degree of conformity.For each vehicle in the fleet, the reduction in emissions for a future technology package can be estimated using RSEs associated with the initial and final technology packages, and considering the particular vehicle’s weight, road load, and power. As part of the effectiveness assessment based on weight, road load, and power, this paper will also examine of the effect of performance changes in the vehicles.