Isolate 12 V analog signal from 5 V ADC
You are quite correct that spikes over 100 V can appear on automotive supplies, mostly due to starting circuits.

A resistive voltage divider of say 12 V to 3 V should be sufficient if followed by a 3.3 V zener and a 1 nF capacitor to get rid of the overvoltages.

See the following example showing the basic idea after some comments above:







simulate this circuit &ndash; Schematic created using CircuitLab