This was posted to the 80/90/CQ/Cabrio forum on Audi World 12/11/2002 by user BigMouse...

Ever since I swapped the 20vt in, my tach has been reading high. I found the solution to the problem today. If you remove the tach assembly from the cluster, there is a very small potentiometer inside where the electronics are. It is on the side opposite the coolant temp gauge. What I did:

Remove the board.
Remove the tach/coolant temp assembly.
Put tach/coolant assembly on board and take to car.
Carefully plug in the yellow and the blue plugs and hold the tach onto the board. (Make sure you don't tweak the board, it may break if you do).
Attach an external, induction type tach to a plug wire.
Rev the engine to a decent speed (2500-3000rpm) and hold it there.
Turn the adjustment potentiometer so that the tach needle matches the value your external tach shows.
Your tach is now calibrated and you may reassemble everything.

Note: Setting at a higher RPM may provide more accuracy. I didn't because my engine was still cold.

I'm glad I've finally figure this out. Not knowing how high my engine was revving was really bothering me.