johnerrington:
@arnakazim
On my old rally cars (1980s) we used to use an electronic ignition system.The simplest goes between the distributor and coil and isolates the points voltage from the coil voltage.
That would greatly simplify the implementation of a rev counter.
I want my system to be pluggable without modifying anything, so the car stay as stuck as much as possible. But for sure it would be easier that way.
raschemmel:
FYI, the R5/C1 RC LPF time constant is 5mS.
Forgot about the existence of the RC time constant... 
That's how you choose the capacitor value.
From the thread I linked in my previous post (A tachometer (REV counter) using a ugly signal coming from the ignition coil - #30 by sjirafje - Project Guidance - Arduino Forum), using a similar circuit, they obtained a clean enough signal to work with without using a capacitor. So as @JohnRob suggested, I think it could work without one.