Has anyone ever calibrated a stock temp gauge in a 65 to see what it was reading in degrees at different places? Say the gauge is at 1/2 of its travel, is that 190, 210? OR, when does the stock gauge indicate that the engine is too hot?
Over the years the wiring and connectors build up a certain amount of resistance which can throw off the reading a little. Each car is going to be different. Plus the sending unit might be a parts store replacement and will vary a little one way or the other.
If you want accuracy, buy a quality aftermarket gauge (or gauge set if you also want to know oil press and voltage)