I messaged the guys at tach match best response I received to date.
Recon I will try this route .
Keep you posted here is there reply to my enquiry
Hi Neil -
Indeed you do have a current driven tachometer. The TachMatch TM-02-I-Drive unit will operate your stock tachometer.
My understanding is that the RB25 has a tach output signal on pin 7, which is an open-collector output. Being open-collector, it needs a pull-up resistor to get the signal operative. I would recommend a 1.3k ohm pull-up resistor for use with the TachMatch. If you wish, I could install one inside the unit for you at no extra charge. I'm not sure about the maximum sink current on the RB25 pins, I've seen on the internet that folks use 1k resistors sometimes, so I'm presuming that this slightly higher value (1.3k) should be ok, it loads the pin less than 1k. I can't find the actual spec anywhere for your ECU.