When the plastic gear fails, it can result in various speedometer-related problems, including erratic or no speedometer readings. In some cases, the speedometer needle may bounce around or become stuck at a certain speed. The issue can also affect the odometer, making it inaccurate or non-functional.

To resolve this problem, Porsche enthusiasts often replace the original plastic gear with an upgraded metal or brass gear, which is more durable and less susceptible to wear and tear. This replacement typically involves disassembling part of the speedometer assembly, which can be a delicate operation.

In summary, the most common cause of speedometer issues in older Porsches is the breakdown of the plastic speedometer drive gear due to its material composition, exposure to heat and friction, and the natural aging of the vehicle. Replacing the plastic gear with a more robust material is a common solution to restore the speedometer's accuracy and functionality in these classic cars.