spurs range in size for the GT-4 and the biggest for first is 49T. also check the track width at the rear too see which car you have. it looks like a pro version. track width should be 200mm the W version i think is 190mm the dog bones are different lengths and the pivit balls are different between the versions. the rtr has an extended 2 speed shaft to allow for a pull start. the side belt runs outside of the radio plate on the rtr.
Serpent - Walter RC
Serpent E Power ..... Serpent 747 Reds M3T
Serpent 966-TE Reds M7T .... S411ERYX Speedpassion 3.5T, T-Shox
Serpent 811-TE REDs R5T