Hmmm... I don't think it can be done, and I don't know why the pixel-based ruler behaves as it does.
I assume the pixel ruler is there for Web work because, otherwise, I can't see the point of having such a ruler in a vector application.
You could use gridline every 108 px and 9 subdivisions if you want the gridlines to line up with the ruler ticks.
Let's hope someone else will elucidate.
=-= Harron =-=