Interrupts from a 12v square wave signal... How to reduce voltage to 5v?

I am building a tachometer for a car, and the signal produced by the Engine Computer is a 12v square wave for the RPM signal. How would I go about using this signal to input into an arduino pin? I have read that the max input voltage is around 5.5v, so I am guessing my current 12v square signal will fry the arduino. What is the best way to reduce this voltage so that I can use the wave signal and count the interrupts for my tachometer signal? Thanks

Use a voltage divider or an opto-isolator.