In order to characterize the resistance to surface pitting of gears subjected to contact fatigue cycles, test campaigns have been carried out on toothed rings in through hardened wrought steel 42CrMo4, meshing a case-hardened 17CrNiMo6 pinion. It was observed that due to the operating conditions, low speed, and grease lubrication, significant strain hardening developed on the involute tooth flank surface and subsurface. This resulted in enhanced material properties against pitting and consequently much higher allowable stress levels than those given by the ISO 6336 series of standard. The 42CrMo4 load capacity was increased by 38% at surface pressure (+90% in torque) compared to the expected results from the ISO 6336. Similar tests carried out on a through hardened wrought steel 30CrNiMo8 gears lead to results of the same order of magnitude. The purpose of this study is to develop a chain of numerical models and methods fed by material tests, in order to improve the accuracy of the estimation of the load capacity of open gear pair with hybrid material (surface case-hardened pinion against through hardened wrought steel). This new approach shows, explains, and predicts the increase in the capacity to withstand surface pressure by comparing present results with rolling contact fatigue lifetimes obtained during tests on several open gears submitted to different mission profiles. This study made it possible to reproduce how and under what conditions the work hardening during running-in reaches required depth resulting in a beneficial effect on the resistance to pitting.