Never use zener diode for an analog input. They leak current. For an analog input, use clamping diodes.
The attached schematic shows how to use clamping diodes, an extra 1k resistor is used to protect the Arduino internal clamping diodes.
R1 : the value sets the range of the voltage to measure
R2 : for example 4k7 or 10k, determines the impedance of the circuit
R3 : 1k to protect the internal clamping diodes.
D1,D2 : The 1N4148 are fast and good for this.
C1,C2 : optional capacitor for example 10nF or 100nF, use none, one or two of them, according to your preferences.
I use a circuit like that for my Geiger counter to protect against voltage of 400V.
For a car, I think that I read once that a circuit must be able to withstand a spike of 120V.
Don’t forget the GND. If the GND can have voltage spikes as well, then it can’t be connected to the Arduino just like that.