Here's a follow up on this issue.  I have a $50 Bosch electric tach in my '55 TBird. I bought it from Summit and it fits in the original tach hole.  I changed it when I switched to a 1957 distributor without tach drive.  It worked perfectly until I installed a Pertronix I module and Flame Thrower coil. It worked OK until about 3,500 RPM.  After that, It would jump up to 6,000 rpm plus and I knew it wasn't reving that high.  I'm a decent engine builder, but not that good.  Anyway, I went on the Pertronix website and they told me what to do and it was an easy fix.  I installed a 10K 1/2 Watt resistor in the wire leading to the tach.  It works perfectly now.

Never had an issue. 



"Came too close to dying to stop living now!"

I bought an Autometer combination electric speedo/tach combination and tach was badly erratic with Pertronix. Pertronix said to use a shielded wire from coil to tach, didn't help. I converted back to points, still erratic, so I sent the gauges back. On my flathead with a scruby distributor/ Pertronix I had an erratic tach (Omega Kustom from Speedway). I had a ballast resistor mounted from the points system. I ran the coil for the pertronix behind the resistor and put the power lead for the pertronix ahead of the resistor. That worked except for a little bobble in the needle at idle. 

EDIT: The site automatically substituted "scruby" for c**vy. 

Ever since I swapped out my points and condenser for a Pertronix I unit, my electronic tachometer has gone crazy. The Pertronix website says to put a resistor or capacitor in line with the wire going from the tach to the distributor.  It also gives a number of options. Anyone have any input into this?  Thanks!