Sensor in cooking oil test. These reading were taken as the temp dropped. The gauge seems to have some damping of needle moment. This was done with engine running to have the same voltage and ground drops over the circuit.
260 deg. on gauge = 249 deg. F on sensor
7/8 deg. on gauge = 232 deg. F on sensor
3/4 deg. on gauge = 220 deg. F on sensor
5/8 deg. on gauge = 211 deg. F on sensor
220 deg. on gauge = 200 deg. F on sensor
3/8 deg. on gauge = 188 deg. F on sensor
1/4 deg. on gauge = 175 deg. F on sensor
So assuming my thermostat opens around 190 that tells me the sensor I used for testing is different or something is faulty with my testing. Of course it could be my thermostat is faulty or someone stuck in the wrong thermostat. Also my fans kick on with the otterstat a little below the 3/8 gauge mark so that says the otterstat kicks on about 185 deg.
I'm assuming the gauge is a calibrated instrument since there is a calibration resistor on the gauge terminals. Maybe they did a crappy job calibrating or did the calibration for full scale. I would have done calibration at the 220 mark (half scale).