Temperature sending unit

Converted my 1955 Dodge Custom Royal to 12 volt system. The temperature gauge on the dash is reading very hot although the actual temperature is about 185 degrees. At this temperature the gauge should read in the middle. Will the gauge work on 12 volts without a resistor? The gauge is currently on a resistor to lower it to 6 volts but I wonder if this is affecting the gauge. The sending unit is advertised for both a 1955 and 1956 Dodge which I would think means it will work on a 12 volt system since the 1956 Dodge was 12 volt. Any ideas appreciated.
Author: ole55dodge