In this paper a computational fluid dynamics (CFD) code is applied to three-dimensional simulations of windage power loss generated by spur gears rotating in air. Emphasis is placed on the simplification which can be made in the numerical approach in order to gain cost and time needed for reaching a converged solution when using both fine mesh and important angular velocity. It appears that the similarity of the flows which are observed in the vicinity of both gear and disk sides is helpful. It is finally possible to compute the flow pattern near the teeth without simulating the contribution of the sides when appropriate boundary conditions are applied at the inlet of a truncated computational domain. The predictions of windage power losses using this original method are satisfying.