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:

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