how do I calibrate a temperature gauge?

[magpies]

While on Jools rolling road it was noticed that the car temperature gauge was reading very high compared to the readings on the rolling road VDU and confirmed by the cut in /out of the electric water pump and fan controller.

When the Rolling road and fan controller were reading 85degC the temperature gauge was reading halfway through the red (approx. 110degC) I believe the oil temperature settled at about 90.

So I think I have the wrong sender but don't know which would be the correct one. Would it be possible to fit a resistor instead? If yes has anyone have an adjustable rheostat I can borrow? I'm in Tesside

Obviously I only need it reasonably accurate about 90 - 100degC

[magpies]

The ecu has a separate temperature sensor which is adjacent to the gauge sender. I have purchased another sender which I have been told should be correct. Has agreen collar instead of the brown one.