I hope this will not be a confusing answer.
The above formulas, while absolutely correct, aren't much help. You would have to know what the resistance per foot is for each wire type, the tollerable voltage drop, the power dissipation capability for the wire, etc.
For your house, unlike your car, there are codes that must be followed by law. A 20 amp circuit uses 12ga wire and a 15 amp can use 14ga. That's solid core copper wire, for runs of a hundred feet or more and under high constant load. If your car were wired like that, it would weigh a lot more than it does now!
With the exception of your spark plug and coil ignition wires, voltage is not an issue. Everything else in your car is nominally 12 volts. It is the CURRENT that will determine the thickness of the wire. (The voltage would determine the thickness of the insulation.)
The things that use the most current in your car are your lights, horn, power windows, heater fan, AC, etc. If you look at those wires, you should find that they are 16ga to 18ga stranded wire. Stick with those and you should be fine.
Hope this helps.