If you want to eliminate your speedo cable and correct for ratio changes you can use a product like this:
http://www.atrol.com/cablex.htm
They also have the electronic signal generator that connects to the transmission.

__________________
Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT