It is best to test and check calibration of these on bench. Now that it is in vehicle pull sender and put in cup of hot water with a thermometer you know is correct. Two or three styrofoam coffee cups work well and do not cool down very fast.

Since sender is basically a resistor whose resistance changes with temperature the instrument is an ammeter whose face is cal'd for temperature. The sender must have the correct resistance.

A 0 to 100 (200??) potentiometer / reostat would work well to check the gage.

Dan Bentler