If the gauge is out of calibration as often is the case as the variable rheostat within the sending unit such as fuel, water temp or oil pressure may be off...
My solution is to use a100 Ohm rheostat with (2) clip leads, connect the rheostat leads in series to 1 side of the gauge...
Now adjust the rheostat to vary the supply DC voltage(5V) to the gauge as to obtain the target value, next using a multimeter measure the value of rheostat setting.
Final step is to add an external 2W resistor in series equal to the measured value of rheostat, so now the gauge will be calibrated.
May sound complicated but actually the practice is quite simple..


Just my $0.02...