On some motorcycles, the electronic tachometer gets its input from the output of the electronic ignition. That's the same output that feeds the low side (primary) of the ignition coil (transfmormer). This output comes out as pulses.
To be used as input to the tachometer, the pulses are fed to a frequency to voltage converter, such as an LM2917 IC chip, which converts those pulses into a varying dc voltage that varies in proportion to the pulse frequencies. Of course, the pulse frequencies are in direct proportion to the engine RPM.
Once those pulses are converted into varying dc, the tach circuitry just reads that varying dc as a volt-meter would. The LM2917 chip was made specifically for this purpose, and Information on the chip can be found on the internet. Here's a location: [u]http://cache.national.com/ds/LM/LM2907.pdf[/u]
However, counting those pulses is a problem, because the ignition coil causes a lot of ringing and distortion and will give a counting circuit a lot of problems to deal with, which results in miscounts, discounts and no-accounts in the kings court. ;D(old joke). I know this, because I tried to do that, and it didn't work so good. But, I did try the LM2917 route and it worked for my purposes.
Patrick