RPM Meter

Hi all,
I have a arduino mega 2560, i wanted to make a RPM meter for my car can anyone help with what will i need and what should the circuit be like?

Do you know the voltage of the RPM signal coming from the motor or ECU?

You will probably need some voltage converter circuit, or at least some protection circuit.

Or do you plan to use the OBDII data bus to give you the data?

How are you powering the Arduino?

What else is the Arduino connected to? An LCD screen? Have you picked out which screen you wish to use? Does it come with a library for Arduino?