It’s amazing the number of classics where an aftermarket temperature gauge or sender doesn’t read ‘normal’ when it’s fine in reality. Owners know these quirks and many simply choose to live with it, knowing that the needle just before the red is fine but any higher is less so.
Sender units and gauges are matched pairs and it’s rare to get perfect readings from a random combination. However, if your combination over-reads, we can suggest a cunning way of calibrating it.
Current from the 12V supply flows through the gauge, heating a bimetallic strip and raising the pointer up the scale. The current returns to earth via the sender unit, which is either a solid resistive compound or another bimetallic strip device. To lower the reading, introduce a resistance into the circuit between gauge and sender. We suggest a variable resistor (potentiometer) of about 100Ω. Break the wire and connect the ends to the central terminal and one outer terminal of the potentiometer. The centre of the gauge should correspond to about 80°C for most older classics, so heat a pan to this temperature and drop in the wired-up sender unit. Adjust the potentiometer until the reading’s correct. It’s unlikely to be correct across the scale, so also test at 60° and 100° and mark the scale or the glass.