With the cable loose from the speedometer, try to hold the end of your cable with your fingers while spinning the front wheel. If the cable has enough torque that you can't hold it, it should be good enough to drive the speedo, if not, then start by removing the cable from the front wheel and see if the drive is turning, maybe attempt to hold it with a screwdriver or pliers to see if it is good. If not, then take the front wheel/brake off and check the drive. The drive gear is plastic with some tabs that can get broken off. If the cable is being driven with some torque at the speedo, then just check and make sure the cable is being fully engaged in the drive in the back of the speedo.
