Tire inflation pressure loss is inevitable during tire service time. The inflation pressure loss rate (IPLR) is widely used to estimate the inflation pressure retention performance of a tire. However, an IPLR test is a time-consuming process that lasts 42 days for a passenger car tire and 105 days for a truck/bus tire. To perform a thorough study of the tire pressure loss process, based on Abaqus software, a finite element model was developed with tire geometry inputs as well as tire material inputs of both mechanical and permeability properties of the various rubber compounds. A new method—the ideal material method—is proposed here to describe the transient tire pressure loss. Different from the previous isotropic models, the cord–rubber system is described using orthotropic diffusivities, which were determined through air-pressure-drop tests then applied in the finite element model in this article. Compared with the standard IPLR test, the difference between the tire IPLR test and the simulation result is within 5%.