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, digital) and can be quite a speed increase to its wishes, to the full board computer.
The network can be found to many circuits, here are some links to interesting projects

Here you will find Schematic Circuit Diagram for DIY and further information on Speedometer
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Applications of the speedometer as LM2907/2917	www.national.com/an/AN/AN-162.pdf
Wireless speedometer	www.aaroncake.net/circuits/tach.htm
Digital speedometer with Atmel AVR	www.xdesignlabs.com/digitaltach/index.html