How to add a custom resolution

First generate a "modeline" by using cvt Syntax is: cvt width height refreshrate

this gives you:

Now tell this to xrandr :

Then you can now add it to the table of possible resolutions of an output of your choice:

The changes are lost after reboot, to set up the resolution persistently, create the file  with the content:

Source: https://askubuntu.com/questions/377937/how-to-set-a-custom-resolution