Well it took about 3 months of research on the web to finally break the code!!!!!! I rebuilt my E body stock tach with modern guts!!!
For the first time in 27 years my tachometer is DEAD ACCURATE FROM 0 to 6500 RPM . Total cost was 116 dollars..

Good show!! What does the back look like?

Here is a pic of the back

Timely thread.....Planning on sending my whole Rally cluster to Tachman this winter.

Been there done that with the replacement circuit board. The new circuit board helps but does NOT replace the 50 + years old springs and major components. What I did was replaced EVERYTHING with VDO internals that were produced in 2022.. Makes a HUGE difference as the tach is now dead accurate throughout the entire rpm range.

what kind of adhesive did you use to attach the vdo guts to the factory face plate ?

I used hot glue to get it positioned then JB Weld epoxy once in place

How are you comparing the gauge reading to the actual rpm? I'm assuming a digital timing light with readout or maybe a second un modified gauge? Looks like the unmodified gauge's sweep to 6500 is different than the rallye gauge's 6500 location?

I'd think twice before sending anything to Tachman.
Absolute worse vendor experience I've ever had.
There's a reason he doesn't accept credit cards, you have no recourse for his terrible service.

It's all about how I positioned the VDO internals.. If you carefully look at the VDO tach face in the picture you will see that it's identical to the sock E body rallye gauge from 0 to 6500. I'm not claiming to be the first to do this modification. Some very high end shops in California do this exact modification for a lot more $$$ than what it cost me to do it myself.. I tested my tach against a Autometer ultra-lite II 0-10,000 rpm tach.. Dead nuts on from 0 to 6500..

Looks good, you should do a detailed write-up to help us out.
What is the exact model PN for the donor tach?

Unless of course you're going to start a side business

Now we need a rebuild procedure for the B-body postage stamp tach!!