It may be the transducer on the top external part of the gearbox final drive (I had one fail, but the speedo would bounce up and down ot a crawling pace as one coil had failed).
But it's more likely the plastic gear ring has broken which requires a complete gearbox strip and rebuild....
Also, the car engine ecu uses that speed signal for itself and will start to degrade the driving experience/performance after a period of time (days/miles)