Alfa Romeo Repair: temperature reading, gauge reading, temp gauge

Question
I have a 146 1.4 twin spark 1999 model and one year ago i noticed that the temperature gauge did not show exactly 90C degrees centigrade. It was 75-80 (estimated by eye). I thought that the thermostat was faulty and changed it. After that the temperature reading on the gauge is always over 90, about 97C (estimated by eye). I checked the cooling fluids and they are ok. I also checked the real temperature of the fluid on the tank with an electronic temperature and its temperature was 89C, with a hot engine after driving for a while. I also changed the temp sensor which is located on the thermostat but the problem persists. Is there a way to read sharp 90C?

Answer
I trust that you immersed the temp sensor in the coolant and did not read the outer surface. In that case, be happy with the result or experiment with various resistors inline to the sensor. Sometimes the auto engineers purposely have a bias for the gauge reading. But, my own alfa gauge corresponds very closely with the actual fluid temp. However, my  ford pickup temp gauge was very misleading so I went aftermarket and now it is very accurate.  

I say the above to illustrate what options you have. I think that a high reading for the temp will just keep me alert and prefer it to a low reading.