[lunarhighway]

race bicycle tachs use a magnet placed on a spoke and a sensor that detects every pass, perhaps a little magnet on the cam belt gear or whater else turns with the engine like the water pump perhaps, than again the alternator signal, could skip this step.

you might be able to find an electronic kit, that is able to do the math. these educational kits usually come with instructions so you just need to know how to solder basically

you might also be able to find an electronic tach kit, wich will be set up to be calibrated etc (unlike stock tachs with might require some hacking)

just some random thoughts...