In this study, the dynamic response of a spur gear pair is analyzed using a novel nonlinear approach. The actual rolling motion and gear engagement are simulated through a set of reduced-order models obtained in a pre-processing phase, using the minimum number of master’s degrees of freedom without loss of accuracy or generality. Gear body flexibility is included by means of a refined finite element model, while all sources of nonlinearity are retained and no geometric simplifications are introduced. To reduce computational cost, the time-varying meshing stiffness is precomputed and applied according to the instantaneous loading conditions. Contact loss and recovery are considered, with reconnection events modeled as vibro-impacts.