Does your car sensor output a value between 12V and 14.3V?
You can reduce higher voltages to lower voltages relatively easily using a voltage divider.
The AREF pin is for setting an (optional) external reference for the A/D converter other than the existing options of 5V and 1.1V. Any input voltage values outside the range of 0V to AREF (either 5V or 1.1V) will not give meaningful readings, and values above 5V or below 0V will damage your board.