yr sc400 will have a 4 pulse per rev speed sensor
some toyotas have 20 pulses per rev but im pretty sure sc400 has 4
so what u do is goto autometer and get an electronic sensor that fits yr th400 to suit an autometer electronic speedo
see if u can get a 4 pulse type but if u cant u then have to get a pulse divider/multiplier and then adjust the the unit to get correct speed
i can get stuff u need here but u prob pay more than if u bought them in usa