smason

Well I have an update! I've been working almost non stop so I've been very busy. But this will be the finale - I think.
I have just been driving with the tachometer which is less twitchy, and no speedometer.
Well as legend has it, last night I was driving to work, and the speedometer came to life!
I changed nothing. I did nothing. The RC circuit works and it is kind of accurate. The multiplier works but the speedo is just 'out' at higher speeds. No fault of the G4.
 
This is a classic representation of a USDM 240sx and it's troubles.

Just a thought, when sending that aux signal through teh RC circuit through the dash are you sending it through the wires which used to go to the gearbox sensor or through the factory speed input into the ECU wires? In my Stagea the speed wires to the dash run the length of the gearbox and go in to a plug in he engine bay then through the body loom to the dash, a modified version of this signal is then sent from the speedo across the car t the ECU as a speed input for the ECU. This means that there are two speed wires for the dash cluster, one in and one out (+ the signal grounds of course).

If you have more than one output set to tacho you will run into problems, turn off that second tacho output

Yes that cap will be fine.  Any ceramic or tantalum (maybe easier to find in 1uF), 20/30/50V will do it.

The 0-1V signal is usually a simulated narrowband signal.  Im pretty sure they have a 0-5V wideband output wire as well which would be the better option.
 
The ECU can do a calibrated speedo output.  You will need a speed input from a VSS.  The output to the speedo will need a small circuit to make it compatible with the nissan speedo which expects an AC signal.

 
You will need to assign all the inputs and outputs to match your wiring.  Apart from those and the trigger offset you will likely also need to adjust the master fuel number to get it to run decent also.