dc42:
I'd use a small signal NPN transistor. Connect emitter to ground, collector to the Arduino pin (configured as an input with pullup resistor enabled), and base to the tach signal through a 100K resistor. The resistor will protect the transistor from the positive going spike, also from any excessive negative-going voltage (although from your scope trace it looks like it only goes to -2V, which is insufficient to cause the emitter-base junction to avalanche anyway).
Part number suggestions for that NPN transistor? It needs to work with the input tach signal of 2.4v
Far-seeker:
A diode before the Arduino's input pin will keep a momentary glitch from ruining a pin, and possibly greater damage, to the ATmega IC which would cost more in both time and money to replace than installing the extra diode. Therefore, in my estimation, it's a worthwhile addition even if not absolutely necessary.
I agree with the diode suggestion. Better safe then sorry.
Suggestions for a suitable one?