Nut that attaches to speedometer can be removed by disconnecting speedometer cable at front wheel sliding nut off of cable. Cable Ferrell that came with new gauge should just need to be slid on to VW cable and attached to speedometer and then re install cable at front wheel. If dolphin Ferrell does not fit one can be found at Texas Industrial Electric .com.
Or a nut that fits the dolphin gauge could be slid onto the cable and tightened as a Ferrell.
If your TDr is not a VW powered one nut on cable can be removed with hacksaw and new Ferrell or nut can have a slot cut into the nut so it can be attached over the cable and to speedometer.
Good luck.