For all of you electronic experimenter types out there ...
I was looking at the data for this IC normally used and recommended for tachometers ... and, lo and behold, it has other capabilities that come to mind for 911 applications ... only a few additional components are needed to have a complete, working 'black box' ...
Here is the site to download the datasheet and application note AN-162:
http://www.national.com/pf/LM/LM2917.html
Different versions are available for use with points [single-ended input] or magnetic reluctor inputs [SC and Turbo distributors] ... so if you have a non-functioning tach or speedo that is already apart, here is an easy way to revive it with some modern IC technology and just a bit of fabrication on a universal breadboard -- if you aren't inclined to make a circuit board from scratch! There is also a LM2907 version, but the LM2917 version is preferred because of a built-in Zener to regulate Voltage to the circuit.
Just a few 911 applications:
1. Tach repair -- replace old VDO board needing unobtanium parts ...
2. MFI Speed Switch/RPM Transducer/Speed Relay ... replace an ancient Bosch board that has seen better days ... at very low cost!!!
3. Adjustable rev-limiter/shift lamp ...
4. Speedometer repair -- for electronic versions, only -- easy calibration for new/different tire sizes ...