The tach on my 82 rx7 still moves, but its very wierd. I t jumps around alot. Sometimes it is somewhat representative of engine speed, but reads higher. When I put the clutch in, especially from 3rd to fourth, it jumps to like 5000 and hangs there. The car seems to drive fine. But i have no idea how the tachometer works or how to fix it. Thank you for your help.

 

Could be that your trailing ignitor is starting to go south, try switching them and see if that makes any difference.