A Range-Extended Electric Vehicle (REEV) usually has an auxiliary power source that can provide additional range when the main Rechargeable Energy Storage System (RESS) runs out. The range extender can be a fuel cell, a gas turbine, or an Internal Combustion Engine (ICE) bolted to a generator. Sizing the powertrain for a REEV is primarily to investigate the relationship between the capacity of the main RESS and the power rating of the range extender. Worldwide harmonized Light vehicles Test Procedures (WLTP) introduced a Utility Factor (UF) which is a curve used to calculate the weighted test results for the Off-Vehicle Charging-Hybrid Electric Vehicle (OVC-HEV) from the measured Charge Depleting (CD) mode range result, and the Charge Sustaining (CS) mode Fuel Consumption (FC). Therefore, the RESS capacity, the range extender power rating, the control strategy, and the UF are the key factors affecting the weighted FC of a REEV on the test cycle. The aim of this study is to demonstrate a fast approach to develop REEV powertrain concepts. It can size the capacity of the RESS (assumed electric battery for this paper), the power rating of the range extender and meanwhile consider the control strategy and the UF for a REEV, using simulation and Design of Experiments (DoE) tools. For the selected REEV powertrain, a DoE test matrix of the battery capacity, range extender power rating, and control strategy was created. The test cases were then imported into the simulation environment to perform the driving cycle simulations. After that, the simulation results (along with the UF) were used to calculate the weighted FC. Finally, a REEV weighted FC emulator model was created and interrogated using model visualisation and optimisation methods. Furthermore, the weighted FC’s calculated by using different regional Utility Factors were compared and discussed.