This is the optional 48 tooth center spur gear for the Thunder Tiger EB4 S3 buggy. This is an optional part to give the car a little more acceleration out of the corners at the expense of top end speed on the straight (ie: better for smaller tracks). The stock spur gear is 46 tooth.
This can also be used to fine tune the gearing on the Thunder Tiger ST-1 monster truck. When using a 13T clutch bell, this gear will provide better gearing if you are running a high performance .28 engine such as the Novarossi 528XR engine.