Regardless of what your gauge manufacturer tells you, you MUST NOT connect any wires to the the vehicle speed sensor (VSS) on your transmission other than the ones that come with you COMPUSHIFT controller harness. The COMPUSHIFT controller provides a speedometer output that will drive many different gauges and digital dashes. This wire should always be used instead of tapping into wires on the vehicle speed sensor.
Dakota Digital VHX System
The Dakota Digital VHX system provides three wires for the VSS: SPD+, SPD-, and SPD SND. Connect the speedometer output from the COMPUSHIFT (purple/white) to the SPD SND input (white) on the VHX system. Leave the other wires disconnected. Calibrate the speedometer per Dakota Digital instructions.