You need the proper sending unit for your gauge and then verify it's reading accurate with a good, accurate, heating and air conditioning pocket thermometer scope
I had a high dollar racing 2 5/8 mechanical Auto Meter gauge that red 140 F when the circulating coolant in the radiator was body tempature shock I bought a good pocket thermometer and it read 98F when that stupid Auto Meter racing gauge read 140 F down puke Same thing on two new Stewart Warner mechanical industrial temp. gauges runaway
Always trust and then verify when in doubt up twocents

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)