I made these gears to reduce the gear ratio for my Ultimaker to 3.75 : 1 (6.125 default). It makes the retractions completely silent when paired with SilentStepSticks stepper drivers (and operated at <20mm/s). Don't forget to change the Estep parameters in your Controller or firmware to 530.1 from 865.8. For best results print at <50 mm/s and turn on combing in retraction.
Turn on combing for retraction