Found a fix for 240Z Temp Gauges that always read past half, even after 3 row High Efficiency radiator, Nissan 170º T-stat, new Nissan temp sender.
Ok the middle of the gauge is approx 185º, always checked with thermometer in radiator and compare what the gauge in the car is reading, in my case I had 2 240Z's at my shop the both showed inaccurate readings at 185º as the gauge was past the M in TEMP.
Also check a 280Z with thermometer in radiator to verify its accuracy and its gauge was right on the money.
Anyway I installed a variable resistor between the water temp sending unit and the yellow feed wire from the gauge and adjusted it accordingly. . . . .Problem Solved.
Here are some pics
And yes I know this is a 10+ year old thread, but maybe it will help somebody out that's frustrated with constant high temp readings.
Joe
Operation Z