This chart:
http://www.powerstream.com/Wire_Size.htm
shows 10GA as good for 15A for power transmission and 55A for chassis wiring (don't know what that means). I don't think I've ever tried to start a car with the lights on (Driving 101) or anything else that would put additional load on a battery. Hey, I even turn off the AC in my wife's car rather than add that load.
I thought about 8GA, but Ace Hdw didn't have any terminals for wire that size.

A VW guy trying his hand at a '59 LB fs. Lots to learn, but it's about the journey.