If the speedometer uses an A/C signal you should connect to both VSS wires. In order to use one you need to connect the other to ground. The pcm will lose 1/2 of the speed signal if this happens.

Got it.

Looks like input to OE speedometer is 4000 pulse per mile. Your math should look like this:

4000 pulse per mile / (tire rev per mile * gear ratio ) = speed signal output pulses per...

Can you can post the year/make/model of the vehicle(s) that you're working on or that donated parts? Over the years GM has used a few different strategies to drive the speedometer.

Around 94 it...