The EMI can arrive to your circuit by several ways: Power supply, The wires that connect the sensor you use to get the RPM(?), and trough the air (EM fields). Some suggestions:

1) If you use tha car battery to supply your circuit, include a protection circuit between the input (probably 12V) and the voltage regulator: an inductance based filter and a spike suppressor (for example a transzorb). Remember that there can be transient voltages ranging from about -100 V to +100 V (spikes) or even more in the suppossed 12 V line. And DC voltages from about 4V (when starting the engine) to 16V when the engine is running at high speed.

2) Enclose your circuit in a metal box (alluminium, for example), and leave a little hole to pass the cables.

3) Filter the signal that comes from the external sensor just when it enters the box.

If this fails, try placing your circuit far from the engine, supply it using an auxiliar battery (not the car's battery), ...

Alfredo del Rio.