This fundamental question baffles me as well, however, I'm quite convinced that components such as the radiator and the clutch fan do affect the operating temperature.

The SD runs at 95°-100°C. now..........no matter if the ambient temp is 85F. or 60F. The brand new thermostat runs exactly the same as the 20 year old thermostat. The chances of the two of them being NG are zero.

I have the suspicion that the difference in location of the sending unit for the gauge and the thermostat is the issue here. If you notice the gauge on downhills, it drops to 81°C. or so. Why? The thermostat would close all the way down and the temp would be maintained close to 90°C. But, this does not happen on the gauge. The only possibility is a difference in coolant temperature between the gauge and the stat.

With a marginal radiator and/or a marginal clutch fan, the temperature swing on the gauge seems to be far greater than one would observe with brand new hardware. Some members have stated that the gauge sits at a rock steady 85°C. despite the load on the engine. These folks have installed a brand new radiator.

I don't fully have the answer to this conundrum, however, I'm positive that the thermostat is not the culprit.

I'll be searching for a solution for the SD as the higher temp. is starting to annoy me.