Having been in the trade for 35 years, I have to add that electrical power travels along the outer surface of a conductor, NOT within it. That's why fine stranded welding cable has such high current carrying capacity - there's hundreds of small copper strands moving the current rather than 7 or 21 large stands such as more stiff conventional cable. The wire's flexibility is a side benefit.
Stinger is a place we order a lot of stuff from. Even their #10 and #8 has many times the conductors as the typical stuff you get at NAPA. We use a lot of #8 and #6 in cop cars for main battery runs. (To the siren, radios, etc.)
I have #0000 for the front winch cables, and #00 for the jumper cables/rear winch feed cable. That's fine strand welding too. It's expensive, but worth it.
On edit: Ditto what Erk469 says about solid core wire. CAT5 might seem like a good idea in a vehicle, (It's cheap) but it will fatigue out and break. If you use small signal wire for switching relays etc. use stranded wire instead.
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