Making sure all your gauges read correctly can be a tricky business sometimes after some changes have been made to your vehicle electronics or fuel tank/instrument gauge etc...or even some case some retrofitting can couse some inaccuracy in gauge readout.
In some case, top of the range VDO fuel gauge might be supplied with built in calibration function but most of the kits are supplied with matching sender unit to save the hassle of calibration. However sometimes it can be more hassle to change out the sender unit to make it fit on the original fuel tank.
There is an easy way to modify your standard (original fitted to your car) gauge to make it adjustable.
In principle, the fuel gauge is an Ohmmeter with a indicator dial showing Full-Empty scale printed on it rather than a resistance read out.
There are 2 situations whereby the fuel gauge does not read correctly.
Out of Calibration Reason 1
The resistance range of the sender unit is wider than what gauge can indicate.
What normally happens is that even if you fill up the tank to the full the gauge will not point all the way to the full level and by the time the tank is 1/2 full or even 3/4 full the gauge shows empty which can be a bit of pain.
Commonly used configuration is that when the fuel level is full the resistance will be low and resistance value increases as fuel level decreases in the tank. If the gauge read out range for example is between 80ohm to 10 Ohm then when the tank is 2/3 full with 260 - 20 Ohm sender range the gauge will read empty even though the tank is 2/3 full.
The way to fix it is this.
Purchase a potentiometer with range of 10000-0 Ohm with at least 3W rating and connect between the the fuel sender unit terminal and ground as shown in the diagram below. Most cases the easiest way to do it is to splice the wire and join the wires together. It can also be soldered to the instrument panel however it is not always practical to pull out the dashboard so take your time and make sure to set it up correctly before you reassemble the dash.
Out of Calibration Reason 2
If the resistance sensing range is wider on the gauge than the fuel level sensor wich is the reverse of above.
What normally happens s that the gauge will read half full even though the tank is nearly empty in which could cause some problem when you are on a public road.
This is due to the fact that the fuel sensor (sender unit) is letting through too much current.
However it can be fixed by bypassing the 12V terminal and the sender terminal with a same 5k - 0 Ohm potentiometer with minimum of 3W rating.
Once the potentiometer has been installed in place the you can now empty the tank to find your empty read out.
Basically twist the dial on the potentiometer until the gauge points to the correct level and there you go.
When you calibrate make sure that it is done when the engine is running at least 2000rpm and alternator is in charging mode. If not, the voltage variation will lead to inaccuracy and the indicator varies quite a lot between 12 ~ 12.4V from battery voltage alone vs 13.8~14.2V from alternator voltage.