Although in almost every vehicle already a tachometer is installed, it can be a worthwhile project to build a tachometer itself.
You learn different worlds Kenen (mechanics, engine technology, analog technology, digital technology) and can expand quite a tachometer according to his wishes, to the full board computer.
The network can be found to many circuits, here are some links to interesting projects

Here you may find Circuit, Basics, Schematic Diagram and DIY Kit components for Speedometer