Check the bezel of your gauge and see if it has a sticker.
Sounds like you have a gauge that has a bezel or sticker from another gauge that starts reading from the other side...
You can test temp gauges and thermostats by boiling water on the stove and a needle type cooking thermometer.
Boiling point of water at sea level is 212.
A 180 thermostat begins to open at 160 and is fully open at 180.
Sounds like everything is working and you just have to flip the numbers written on the gauge.
Never top off the overflow tank on radiators. Just cover the tubes and a little over.
