Selling my Cable-X due to going to a GPS speedo in the Chev, never used but was wired into car and mounting points were modified to suit
Info is here:
http://www.abbott-tach.com/cablex.htm
Takes a VSS signal and converts to run a cable to your original speedo, 3 wire simple install.
Part number is Cable-X-3012 and comes with both the GM snap on and GM 5/8 Thread on cables as I ordered the wrong one once upon a time.
Retails for $340 US and post, I'll sell for $250 posted in Australia.