Warren,
I have used this chip to repair my tacho. I could not find replacement parts for VDO circuit so I decided to build my own using breadboard. I downloaded the data sheets and also found a useful article at
http://www.rgp.nl/bird/BritishWheels.htm
(BTW there was one typo in the circuit dig, where PIN12 should be grounded also).
To check that it worked on the bench, I had to use a frequency generator, and then adjusted the Pot Resistor to match signal revs with dial reading.
Here are pics of how I attached the breadboard to the rear of the dial.
