Add custom resolution to xrandr

Sometimes xrandr does not show one of the resolutions that is accepted by both the graphic card and the connected screen. If that is the case, it is still possible to add manually the required resolutions by using the following steps:

First we need to calculate the VESA Coordinated Video Timing modes for the required resolution. For example, for a monitor 1680x1050 and 60Hz

cvt 1680 1050 60

This will output the following

# 1680x1050 59.95 Hz (CVT 1.76MA) hsync: 65.29 kHz; pclk: 146.25 MHz
Modeline "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync

Now we add the specified CVT to xrandr. Using the previous example

sudo xrandr --newmode "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync

This will create a Virtual screen in xrandr. Now it is possible to add this resolution to any of the screens listen in xrandr. For example to add this resolution to VGA1

sudo xrandr --addmode VGA1 1680x1050_60.00

Finally, to assign this resolution to the screen VGA1 we can run

xrandr --output VGA1 --mode 1680x1050_60.00

source : http://askubuntu.com/questions/377937/how-to-set-a-custom-resolution