55 Fords used a 6V positive ground system. In 56, Ford switched to 12V positive ground, along with Packard, Nash, and a couple of other auto manufacturers and then all the auto manufacturers changed to 12V Negative ground in 1957. This was done to standardize the electrical systems so aftermarket accessory manufacturers could simplify their inventories, among other reasons. Freightliner and a few of the other big truck manufacturers used 12V positive ground until the mid-1960's. The 64 Freightliner I learned to drive after getting out of the Air Force used a 12V + system.
Jerry
I always thought those 2.5 ton trucks carried 24 volt batteries. (2)
