Arduino and tachometer from 12V car battery gen

If you are using a high enough series impedance, then the internal diodes should be sufficient to protect the chip.

No No no, that is bad bad bad.

Look at the chorus of one of by favourite songs
"Will you still care" by The Crockets for my true answer.

http://www.lyricstime.com/the-crocketts-will-you-still-care-lyrics.html