Voltage Drop Calculator by electrician.com,
and HEAVILY MODIFIED by KyferEz and TheCarPCStore.com
The following calculator calculates the voltage drop, and voltage at the end of the wire for American Wire Gauge from 4/0 AWG to 30 AWG, aluminum or copper wire.
For all applications, but especially car stereo, you want minimal voltage drop due to the wire. The higher the voltage drop in the wire, the more sound you're losing, as your amplifier is seeing a lower voltage at it's input, which means it will produce less power, which means less sound. With me so far?
Now you don't want to go overboard; all wire is going to have voltage drop - that's no reason to use 4/0 wire on a 100 WRMS amplifier!
Using the calcualtor: For car stereo, select, from top down, "Copper", the gauge of wire you intend to use, "12 VDC or 1-phase AC", how long your wire will be when run in the car, and how much wattage OR how much current you intend to conduct through the wire.