Join Date: Mar 2002
Location: Austin, TX, USA
Guage temperatures can vary considerably depending on where the sensor is located and the configuration of your cooling system. Although, your reading seems high. It is very possible for a bad ground or other electrical system problem to cause a misread.
About the only way to know if the guage is correct is to remove the sensor and boil it in water with an ohm meter attached. Keep the water stirred and record several temperature / resistance readings. You can then insert a variable resistor where the sensor would go in the engine bay and see if the guage matches your experimental readings.
Temperature sending units are non-linear. As a result, the guage readings can be accurate around the 200 mark and way off at the other ends of the scale. You'll probably notice that the graduations on the meter face aren't linearly spaced like a pressure guage is. The uneven spacing is supposed to compensate for non-linearity in the sensor to some degree.
Some guages, like the autolite guages, have a calibration adjustment in the back of the guage. Mine are accessed by removing the light bulb. I use a small screwdriver with a length of heat srink tubing around the shaft. I don't know if the adjustment is hot or not. The tubing is just my way of insuring I don't short the guage circuit in the process of making the ajustment. What I recommend is boiling the sensor (while stirring) and measuring the resistance. This will be darn near exactly 212 degrees. The put the equivalent amount of resistance across the guage lead to ground and calibrate your guage for ~212*.