You need a series resistor to cut the current coming in and to isolate that part of the circuit (the one on the RPM sensor) from the inner part.
Then you need ordinary (small) diodes to clamp what does come across the resistor to V+ and ground (i.e. one diode preventing the voltage from getting above V+ by conducting anything above V+ to V+, and a second preventing anything from going below ground - don't get these backward).
Then a second, probably smaller resistor to go into the arduino's input.
All right! Good answer! I can completely understand that and it sounds quite bulletproof - thank you for the lesson.
You can use a zener as a single diode to ground - e.g. a 3 volt zener would clamp below ground to ground, and above 3 to 3.
I did try a zener but It needed a fair bit of current flowing to regulate - 10 or 20ma I think and I wasn't comfortable putting that load on my circuit.
Thanks again.