Has anyone gone from a cable driven speedometer to a digital speedometer? if so what sensor did you use? or what did you have to do?

 

Yes, I've done it.

The VSS from a FD will bolt right onto the FC tranny.
You need to swap the driven gear and retain the FC part but it's pretty straightforward.

I'm using a Miata cluster and this arrangement is probably more accurate than the original.

 

if you are thinking of a aftermarket speedo- go for a GPS speedo

 

How much is one of these?

 

Auto Meter makes a GPS add-on that you can hook up to most of their electric speedometers.
Auto Meter - GPS

Another option is to use an electric speedometer with a hall effect sensor.
SPA Technique - Tachometer/Speedometer

 

Speedhut also makes them