My 55 has been converted to 12 volts and since day one when the motor is cold the gauge starts out about 1/3 the way up past cold,used a instrument voltage regulator from a 65 galaxie to reduce the voltage. Tried different original 6 volt sending units and a NOS gauge and still the same,gas gauge works fine so what did I do wrong.
They also say not to use any gasket sealer or teflon tape on the sending unit so it gets a good ground to the engine.
I worked on a 55 Ford temp gauge a couple years ago. Gauge would not read correctly, or where it looked correct. Talked with a early T-Bird expert and he helped a lot. ( Internet also helped a lot.) I was helping a friend out who had the car restored by another guy and many things were not correct I found out. First I needed the correct sending unit. I believe 6 and 12 volts are different. You need the 12 volt unit. Second, no Teflon tape or sealant on sender. Third, I had to change gauges, but I can't remember exactly why now. I think the original was broken. After all this, the gauge still did not read where I thought it should. I discovered that the gauge needle is adjustable as to where it sits at full cold or hot or when key is off. Very easy to adjust by removing the gauge and looking at the back side, there is a small access hole with a toothed(?) movable piece that moves the needle . I used a small screwdriver to adjust. I used a laser temperature thermometer to make sure the motor was not overheating and adjusted the gauge to read right where I thought best when full hot. Worked perfectly. The gauge was really just a idiot gauge with no real temperature value, so by setting it to what looked to be comfortable and verifying the actual temperature , all was good.
Vehicles that are 6 volt reduce the gauge voltage to 3 volts. If the battery gets low the gauge will still read accurately. You have used the galaxie reducer to drop voltage, now you need to reduce voltage again. I used 2 runtz reducers in series on my 54 merc. It works fine. Harley
I will get another IVR and add it and see how it does,the gas gauge works fine but not the temp gauge.