if the mechanical gauge is designed to show cold and hot it will move both ways. An
automotive gauge NO. They usually start off at 0 then go to 250 or hot. They will not go
to -0. if they do you have your gauge connected to your A.C. system and if it isn't you
need a new gauge. So, what do you have ? electric or mechanical ? Just looked at your post again. The 50 ford is positive ground and 6 volts. If you have an electric gauge
connected to positive ground it will show cold as the engine heats up. Reverse polarity.
If this is the factory dash gauge you have to leave it positive ground. If you convert to
negative ground, and you probably did that is why your temp gauge shows backwards if it is factory, your gas gauge may also work backwards. It shouldn't but it might. Like i said before. Need to know what your doing. More info.