The spur gear is considered a consumable item and will require replacement from time to time to ensure maximum performance from your model. The clutch bell in a nitro vehicle is made of steel and should last much longer, but it too will need replacing down the road.
Visually inspect the spur gear after every run for signs of wear or small debris lodged between the spur gear's tooth profile. Small stones can be removed from the gear with a hobby knife, but if the spur gear show signs of excessive wear replace the gear before continuing to run the model. It is also important that you periodically check the engine mounting screws and gear mesh.
For Traxxas electric vehicles such as the Rustler, Stampede, or Bandit, spur gear replacement is quite easy. Simply remove the gear cover on the transmission and replace the spur gear by removing the three 3mm CS screws that hold the spur gear onto the slipper hub. Most other brands keep the spur gear readily accessible so it can be changed in a matter of minutes.
If your spur is "worn all the way around" - that usually indicates your pinion gear is meshed too far away from the spur (meshed a little too loose). That may indicate that your motor/engine screws are a little loose. Re-mesh the spur gear and make sure those bolts are tight to ensure it doesn't happen again any time soon!
The two spur gears shown above, show signs of excesive wear and should be replace before running the model.