Converting a tachometer signal into something a microprocessor can understand
Add a clamping circuit using a series resistor, followed by a 5V zener diode to ground.  The resistor value and zener diode wattage should be chosen together so that the zener's wattage can not be exceeded by a continuous overvoltage placed on the input.  In a car you typically assume about 70V is the max voltage you'll see (during an alternator failure).  

In the case of a tachometer input you can use a fairly high series resistance (10k) since you need very little current into your detection circuit.  After the zener add a small capacitor which when coupled with the series resistor forms a low pass circuit.  Feed the signal into a Schmitt trigger input.  You may also want to add a series fuse, and a MOV or TVS diode to add additional spike suppression.

Instead of a Schmitt trigger logic device you might consider decreasing the series resistor (keep the zener) and using a Schmitt trigger optoisolator like the H11L3M from Fairchild.  This will further protect your microcontroller from spikes and surges.