The objective of this study is to conduct a numerical investigation to examine the temperatures in off-the-road (OTR) tires under operating conditions at mine sites. To achieve this, a new mathematical equation was developed based on a modified Mooney–Rivlin (MR) strain energy function, the pseudo-elasticity theory, and the inverse analysis method. This equation was used to determine the internal heat generation rates of tire rubbers. With heat generation rates, the governing equation of heat conduction and the mathematical expression of boundary conditions were further generated to describe the heat transfer in tire rubbers. Based on these equations, a novel finite element (FE) OTR tire thermal (OTRTire-T) model was developed. This OTRTire-T model was used to numerically investigate temperatures in OTR tires at vertical loads from 0.34 to 1.04 MN, hauling speeds from 5 to 30 km/h, and ambient temperatures from −30 to 40 °C. The results showed that a large vertical load (e.g., 1.04 MN) increased the tire rubber temperatures considerably. Tire rubber temperature also increased with an increase in hauling speeds, and the increase became more significant at larger vertical loads (e.g., 1.04 MN). The OTRTire-T model identified an inverse proportional relationship between the rubber temperature increments and the ambient temperatures from −30 to 40 °C. Nonetheless, the rubber temperature in the OTR tire increased relatively rapidly with an increase in ambient temperatures.