Really good technical article over on team camaro.
http://www.camaros.net/forums/showthread.php?t=184196
Apparently, the 67 and 68 tachs have a built in adjustable variable resistor that can be adjusted while the car is running. No such thing on the 69.
I asked this question over there, but apparently no one has tried this. Any reason I can't put an adjustable resistor in line from the coil to the tach? For that matter, wouldn't an adjustable resistor from the batt to the tach do the same thing?
I know, I can send the tach out for calibration. I just don't want to.