The needle resting at 20 mph has nothing to do with the transmission, that's a different issue all together.
The needle can be removed and put back on at "0", it's a press fit. Put a very small prybar directly under the needle hub (do NOT pry on the actual needle or you will break it) and gently pry it off of the shaft, then gently push it back on so it rests at zero. Make sure you protect the gauge face when prying it off.
Mike B
