Where is the sensor ground wire? Its one of the two pins from the VSS sensor in the trans.
Page 8 of the .pdf
http://www.dakotadigital.com/pdf/sgi-5.pdf
"Speedometer will read Tach wire too close to Route the speed signal and tachometer
when the vehicle is speed signal wire. wires away from each other to avoid
sitting still. interference.
Signal In and OUT wires Route the input and output wires away
routed too close. from each other to avoid feedback.
Ground interference. Make sure the speed sensor and SGI-5
are grounded together."