Computational fluid dynamics (CFD) simulations of the thermal environment surrounding an underbody fuel tank in a passenger vehicle are presented. In this study, an orthogonally structured full vehicle CFD model was used to provide the necessary boundary conditions for a body-fitted unstructured CFD model of the underbody region containing the fuel tank surface. This method provides a timely approach to performing this type of simulation in support of rapidly changing vehicle development programs. The results for both idle and 30 mph conditions indicate that the major source of external fuel tank heating is the heat convected to the fuel tank surface from the underhood region of the vehicle. Comparisons of simulation results with test data from a similar vehicle shows a good correlation between the two.