Overvolt an nVidia GeForce GTX 650 on Linux

Hi guys,

is it possible to overvolt a GTX 650 on Linux? I already set the CoolBits option to 28, but nvidia-settings still says the attribute GPUOverVoltageOffset is read-only. I’m using the latest nVidia driver (390.48).
Am I doing something wrong?

I wrote this tutorial a couple days ago. I hope you find it useful:
https://askubuntu.com/questions/7749/how-can-i-overclock-a-graphics-card-from-within-ubuntu/1037178#1037178

I already did that, but I want to change the GPU voltage, too. Is there a way to do that?