I was searching for the way to adjust my 3800 series II in an 88 fiero and I saw where people had said to add resistance to the tach signal, but I never saw how much. I'm just posting this to help out people in the future. I compared the tach to the OBD scan tool I have, and they were very close, almost perfect, with 13,760 ohms, or 13.76k ohms.
UPDATE: Well, after driving for a while, the resistors heated up, and obviously that increased the resistance so the tach was reading lower than OBD. I will update later with the value that holds true after the engine has warmed up. It looks like it will read high when cold, but this will work for a simple, cheap fix.
[This message has been edited by aarwigg (edited 04-26-2014).]