This is the Treal hardened steel overdrive diff gear set for the Axial SCX24.  These fit either the front or rear axles and provide increased wheel speed over the stock gear ratio.  These feature a 14T gear and run approximately 14% faster than stock.  

Note:  Package contains a gear set.  Purchase 2 if adding to the front and rear axle.  If purchasing 1, we recommend installation in the front axle.