[Jim/GA]

As Joe says, the basic advantage is, for any given load, you have 1/2 the amps going through the wires and switches at 12 volts compared to the current flow at 6 volts. Flowing 1/2 the current through a switch or starter motor generates 1/4 the heat. That switch will last longer. That starter motor will not get so hot so fast.

12 volts is also less sensitive to slightly poor grounds or imperfect wire connections. You might lose 1 of your 12 volts with a bad connection, but the light or horn, or whatever it is, will still work.

On a 6 volt system, that same bad connection will lose 2 volts (due to double the current) and only leave you with 4 volts to light the light (it will be dim) or blow the horn (it will sound weak). So, as Joe said, you need to be more diligent with all of your grounds and connections with 6 volts to keep them clean and tight. It can be done. Cars ran on 6 volts for decades.

Personally, I would opt to push start an A with a dead or weak battery instead of jumping it. There is usually enough juice to get a spark (if the starter is not cranking, pulling the battery volts down really low). Getting to the battery on the A to jump it is a pain (under the floor mat or carpet) and jumping to the battery cable under the hood at the starter switch is too easy to accidentally touch a ground and start arcing.