The paper deals with the problem of volumetric wear of timing pulleys. The authors analyze the course of this phenomenon depending on the properties of the belt-pulley friction pair. Timing belts are made of various materials, including additional coating layers, similarly to pulleys. The gear operation problem consists in modelling the pulley life for specific gear materials. The authors made also an attempt to investigate the rate of this phenomenon and its impact on the kinematic efficiency of the transmission gear with timing belts.