Bending strength of injection molded polyacetal test gears is investigated in back-to-back testing using oil-lubrication. To validate state of the art calculation methods, tooth geometries with a variable number of teeth are investigated, maintaining a constant center distance and transmission ratio. To enable testing on a constant level of tooth temperature for variable torque loads and speeds, occurring tooth temperatures are measured and considered in latter testing. Test results show that bending strength of tooth geometries with a higher number of teeth stand higher tooth root stresses acc. to VDI 2736, as the calculated tooth root stress is too high due to negligence of load-induced deflections. A modified method for calculating tooth root stress considers load-induced deflections, allowing to trace back the failures of the different tooth geometries to a common stress level. Therefore, a possible approach to consider the actual contact ratio for tooth root stress calculation of steel-plastic spur gear pairings is proposed.