I'm trying to figure out why the factory tach on my 85 Z28 consistently reads about 300 - 400 rpm too low. I'm wondering if the zero setting on mine is off. If your tach is accurate, could you do this quick experiment:

Turn the key ON, but don't start the car. What does your tach needle read? Zero or less than zero?

My tach reads less than zero under these conditions. When I put a new tach board into my 76 Corvette, I had to set the zero so it would read accurately. I'm wondering if there's something similar in this one as well.