Several things. If you are starting from a 12V car, the 7812 cannot regulate because its drop out voltage is ~2.2V. This is made worse by the diode in series with the input voltage. The engine-off voltage of a lead-acid battery is about 12.6V, and when the engine is running, it will be about 14.2V.
I would be inclined to dump the regulator altogether. Replace it with a 100Ω resistor, and a shunt 47uF 25V electrolytic to ground. The series resistor and shunt capacitor form a low-pass filter good enough to gobble any spikes; the CMOS Vdd can be anything from 3 to 18V, and it doesn't need to be regulated (especially badly regulated
).
For the input, I would just put series 10K, and a shunt 1nF, 10nF, or 100nF ceramic to ground. Which value depends on the expected max pulse rate?