I think you're reading the allowed power supply voltage range. It's true that the onboard voltage regulator can tolerate up to about 20V. However the digital I/O pins are far less tolerant. The Arduino is liable to be destroyed if you connect anything less than 0V or greater than 5V (relative to Arduino ground) to an I/O pin, and the I/O pin could be damaged if you connect anything to it that draws more than about 20mA. Unless you know what you're doing, it would be very easy for you to destroy either the car or the Arduino, and it would only take a momentary mistake to do it.