a small plastic gear with a slotted hole in it
Can you position a optical interrupter photo sensor to read through the slot?
I'm trying to avoid magnetic pickups, hall effect sensors, etc, because the speed gear itself is plastic, and a metal version is not available.
Can you attach a magnet to the side of the gear?
mounting a small servo motor to the gear, and having the gearbox spin the servo
A rotary encoder turned by the gear.