The Office of Transportation and Air Quality (OTAQ) of the U.S. Environmental Protection Agency (EPA) has started work on the next generation mobile source emission factor model, termed the 'Multi-scale Motor Vehicle and Equipment Emission System' (MOVES). MOVES will be the successor to the MOBILE6 highway vehicle emission factor model and the NONROAD model and include ozone precursors, particulate emissions, toxics and greenhouse gases. In the last few years advances in technology have made it possible to measure vehicle tailpipe emissions during real-world vehicle operation using portable on-board instruments. This on-board emissions data will be the basis for the MOVES model. Because the measurement and interpretation of on-board data is a relatively new area, EPA sought input from external experts to provide examples and recommendations of modeling approaches including steps to calibrate and validate their models. Each of the three organizations worked simultaneously and independently under the same scope of work and each prepared its own report. The three organizations were North Carolina State University (NCSU), the University of California at Riverside (UCR), and Environ. EPA also worked simultaneously on another approach. This paper shows each approach as well as the results of the shootout and then discusses how EPA will proceed with the exhaust emissions calculation methodology in the MOVES model.