First generate a "modeline" by using cvt
Syntax is: cvt width height refreshrate
cvt 1680 1050 60
this gives you:
# 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 tell this to xrandr:
xrandr --newmode "1680x1050_60.00" 146.25 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync
Then you can now add it to the table of possible resolutions of an output of your choice:
xrandr --addmode VGA-0 1680x1050_60.00
The changes are lost after reboot, to set up the resolution persistently, create the file ~/.xprofile
with the content:
#!/bin/sh
xrandr --newmode "1680x1050_60.00" 146.25 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync
xrandr --addmode VGA-0 1680x1050_60.00
You need to replace VGA-0 with your monitor connection. Use xrandr --listmonitors.