Helical gears from an automotive gearbox, previously subjected to the surface treatments of carbo‐nitriding and shot‐peening, were submitted to contact fatigue tests. The X‐ray diffraction technique was used to characterize the evolution of different mechanical and metallurgical parameters as a function of gear damage. Particular attention was paid to residual stress relief. A numerical model was developed to predict residual stress relaxation and estimate the most likely localization of contact fatigue crack initiation. The stress–strain laws of the surface‐treated layers were determined by means of two separate experimental methods, based on locally measured parameters. The Dang Van multiaxial fatigue criterion was used to analyse the failure of the gears, taking into account the effects of friction and roughness.