Tachometer from spark plug lead and noise

Yes but with a fixed capacitance, which gets discharged at a specific ratio as a result of the current limiting resistor.
I suspect there is just simply a limit to the speed at which that can work.

Normally the cheap tachos work using a circuit which is connected to the base coil. The kickback from the coils should provide you with the signal. Limit the current with a resistor, limit the voltage with a zener diode, that makes the most sense, like the guy in this topic suggested. Have a look here as well, no zener diode in that, but some de-bouncing capacitors.

The complain from someone that the noise from the car itself will influence readings, does make sense. So maybe start out with a fully separate power-supply beyond the opto-coupler, and see if you get good results.