Bly,
I use the VDO tac also and mine is the electronic one. I was fortunate enough to find a patrol office who had a radar set up and he let me run through at 60,70, and 80 miles per hour and I wrote my tac readings down and then compared them to the speedoimeter speed. Using that and the math both my speedometer and tach were right on at all speeds. Your tire circumfrence and gear ratio also figure into this.
gear ratio= tire circumference X rpm X 60
-------------------------------------
speed X 63,360
You can use this to find any of the unknowns. For example it is hard to read my tac at a 25 rpm mark but at 60 I was showing just over 2500 rpm. The actual rpm was 2525. My gear ratio is 3:27 by the way.
Ron
