Just to be a little bit contrarian, I (seem to) have the opposite issue: except in unusual circumstances (which would probably cause the overheating of a number of your cars, for those who have such a problem), the thermometer on my late 1968, 4-speed manual 280SL stays around 70ºC, sometimes reaching 80ºC after a good run if the car then idles for a bit, before returning to around 70º.
The engine has been completely overhauled after a high regime incident in which a piston and rod where damaged (block and head, including all parts thereof were checked and replaced if necessary, resurfacing, etc.), radiator recored, water pump and thermostat changed.
I have checked several times the temperature of the various accessible parts of the engine with an infrared gun (radiator in and out pipes, water pump in and out pipes, head and block in various locations (left and right side, top and bottom, at different cylinders), heater pipes, pipe to air intake box, etc.) during warmups and after runs, and always found it to be around 85-90ºC (depending on location and after warmup obviously), sometimes 95ºC at the entry to the water pump and near cylinder 6 after a good run. Should we conclude that the cooling system, when everything is in good working order, is almost flawless? No AC to interfere with the normal engine cooling in the engine bay. Where I live is never stifling hot (maybe 35ºC for a few days in late July at midday), very low humidity (continental Mediterranean climate, 500m altitude), and traffic jams are the subject of stories told to unruly children to make them behave or else...
So, in a nutshell, nothing wrong (I think) except maybe for the display in the car. I nevertheless hate to think about this car being used in Poland, Germany, Canada...
Having said that, in case I wanted to fix the display, is the opinion of our distinguished experts on this forum that the problem (if there is a problem) is to be found in the gas/tube or in the gauge itself (I believe the two are inseparable?), is there a tweak (accessible to an amateur like me) on the gauge itself that is used to correct its reading?
Does anyone have a similar experience with the actual performance of the cooling system in their car?
For those who have yet more patience reading about this, I had a somewhat similar issue with my previous old timer, a 1962 JaguarMk2, after I had the original radiator replaced by a more efficient aluminum one, made by ... Mercedes, of all companies (or at least designed by them or for them). The engine would never get warm enough except in exceptional circumstances, which would probably cause other cars to overheat. In that case though, the gauge was displaying the correct temperature, which was really too low. I changed the thermostat twice, nothing would do. In the end I had to restrict the airflow through the radiator to make it less effective! The world on its head, as we say in French (Le monde à l’envers).
For those of you in the Northern hemisphere, keep warm (and safe), in spite of my stories. For all others, keep cool (and safe too)!
