did you measure temp of the thermostat housing when hot? You might be able to find resistance curve for your sensor. basically if you find resistance at 212 (water boiling, corrected for elevation) and freezing at 32. Any where in between you can interpolate the expected result. basically you need to check calibration. just for comparison the following were finding for my temp sending unit. As the temp goes down goes up the resistance in Ohms.
Edit. Had a correction here that just noticed. Resistance and temp are inversely proportional. Type in in correctly on my original post. don't know if anybody noticed.
For 1700 tractor Operating temps:
Cold: sender=690 ohms, T housing = 98 , Radiator= 98 , Head=98 , Exh Man=98
Operating temp: sender=140 ohms, T housing = 172 , Radiator=170 , Head=174 , Exh Man=192
Hot,1/8" , right of middle: sender=92 ohms, T housing = 196 , Radiator= 196 , Head= 197 , Exh Man=210
1/8" left of H: sender=66 ohms, T housing = 210 , Radiator= 210 , Head=212 , Exh Man=250
