From what I have been finding on forum searches.
On a S5 with ABS or Cruise control you can tap the speed sensor wire to get to input into a Haltech ECU. (i have an E8)
Both of which I still have installed.

Question is on the programming on ECU
1. is it a pulsing signal @ 1553 pulses/mi? or other in km?
2. I have a spare Timed input 3 available. Which do I program, Rising or Falling on it?
3. Are the gear ratios the same out the FSM to input into the ECU? (Will be planning Boost control by gear)

I recall I had an Apexi AVC-R installed and had the speed hooked up but that was back in 2003/4..

 

So this does not seem to be working.. the MPH value just keeps increasing.
When I am standing still if is a static high number.

 

Found this out for the E8/E11 Haltech.
To determine vehicle speed, a square wave signal must be applied to the road speed input connector. This connector possesses ground and 12 volt signals for powering a Hall effect or optical sensor. A magnetic or reluctor type signal is incompatible, and you will need to convert the output from this style of pickup to a square wave.

Question is now what type of signal is in use?

 

Found these converters if needed
https://sirhclabs.com/product/vr-to-...sor-converter/
or these
https://sirhclabs.com/product/cortex...ensor-adapter/

 

Good stuff. My tuner recommended using the wheel speed sensors or getting an aftermarket gps one, specially from Haltech. I think there is another option where you can use a miata electric speedo cable and ditch the fc cable one. I have yet to find any more how to for the miata cable switch.

 

Nice job coming back to update others who search this!

 

The G/R wire that comes out of the cluster and is connected to the PS, AAS, and cruise controllers is a digital speed signal. No reason you shouldn't be able to bring that into the ECU, unless the cluster is broken.

 

You should be able to buy a fd trans VSS and swap the gear from the fc into the fd body. From there it drops right in. Part number: R505-17-400B