Here are a couple solutions that may help you connect almost any electronic tachometer to your engine.
There's a really neat gizmo called an electronic ratio box which converts the signal frequency (i.e. rpm) from a magnetic pickup, hall effect, or alternator signal to an output signal of a different frequency (rpm). This allows use of any electronic tachometer. For example, a tach for an eight cylinder engine can be used with a one cylinder engine.
The gizmo is made by Bolec in Australia for VDO (now Continental). Unfortunately, something strange is going on with VDO since they were purchased by Continental. The aftermarket VDO products no longer appear on the VDO website, at least I can't find them. Here's a link to a very small PDF file containing a picture of the ratio box, pin assignments, installation and configuration instructions. You can select one of 255 ratios.
http://www.bolec.com.au/411105_r5c.pdf
I've emailed my instrument man to see if the device is still available and its cost.
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Another solution is a simple frequency/voltage converter you can build yourself for ten bucks or so. It does the same thing as the VDO unit. The device uses an LM2917 chip and a few resistors and capacitors. You can download the datasheet for the chip which includes lots of info and a sample tachometer circuit from the link below:
http://www.datasheetcatalog.org/datashe ... 007942.PDF
FYI
Tachometer Drivers
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