Extremely unstable analogRead() on Arduino Nano when powered from an external source
You can use a branded 5 V charger made for cars. You can directly connect this to 5 V pin of the Arduino.
By doing this:

you are protecting the Arduino from the noise as well as high voltage spikes coming from the car 12 V supply
protecting yourself and others coming in contact with the Arduino or the sensors connected to the Arduino.

You can also

consider filtering the analog data read
protection circuits such as surge protection diodes or a 5.7 V Zener across 5 V and Ground of Arduino