Which of the following 3 wires would I want use to hook up and aftermarket tachometer on a k75? It is a speedo/tach combo so it will already be grounded.

16 - speed signal from ign. coil ( black/blue ) gives rpm signal for tach.

22,23 are from speed sensor in the final drive but the signal is unusable for aftermarket speedos.  You'll need to use the supplied sensor that comes with your instrument.

21 is an OUTPUT, not an input to your instrument.

So 16 is my huckleberry....gracias I would have guessed 21. THANK YOU