The gauge of wire that you should use for your car speakers depends on the power of your car's audio system, the length of the wire runs, and the resistance of the wire. In general, a lower gauge number indicates a thicker wire, which is capable of carrying more current and can be used for longer wire runs. A higher gauge number indicates a thinner wire, which is not suitable for long wire runs or systems with high power.
For most car audio systems, a wire gauge of 16 or 18 is suitable for speaker wire runs of up to 25 feet. For runs longer than 25 feet or for systems with high power, a gauge of 14 or 12 may be necessary. It is important to use the appropriate gauge of wire to ensure that the speakers receive enough power and to prevent damage to the audio system.