1 Mean TT

The factory wire is 4 guage, and the new wire that I would use is also 4 guage, they just have a different kind of insulation.

avincar

The electrons don't care.

RussB

This is one of the few conductors in a car that's big and meaty and isn't fused. For this reason, I'd be very conservative in choosing this wire and its insulation: if it shorts to ground, you'll have fault current roughly equal to the battery's cranking capacity (say, 800 Amperes), which will quickly cause a major meltdown that could easily total the car.

Is the proposed insulation rated to withstand high temperatures, gasoline, and oil? If so, use it.

Me? For this application? I'd use either the OE wire or a good quality high strand count electrotinned stranded wire with temperature and oil/gasoline rated insulation. Welding cable and marine Type 3 wire will work well.

Most industrial electric supply houses sell flexible fiberglas/silicone sleeves that you can pull over top exposed wire insulation to help it resist harsh environments.

Or just buy a new OE cable or pull a good looking used one from a junked car and use it. A good used OE one (from any car, as long it's the correct length with the right terminals) might cost two or three dollars.

1 Mean TT

It is the electrotinned, high count stranded, and I don't really need to worry about a corrosive battery because I'm using an Optima sealed battery. This is for a custom 65 mustang by the way. I just want my power wire to the car audio to match the wire to the starter. It's just a little custom touch that I wanted to incorporate if possible. Thanks for such a detailed response.