At ca. 100 km/h until 108 km/h my steering wheel is vibrating.
I have new sommertyres but it is the same with winter tyres and the previous sommertyres.
My sommer tyresize is not original. . 235/55R18 and I have 20 mm spacers ( track wideners ?? )

Any idea what I can do about it?

 

I guess wheel balancing has been done when tires changed?

 

Yes.
And as i said, it was the same with the old tyres and the winter tyres.

 

It is difficult to say what can be a problem. Although a little question: when the brake disks were changed the last time on the your car? I was always wondering, in such cases, whether uneven rust or stuck dirt/stone inside the brake disk can cause slight vibrations. Because these vibrations will be difficult to distinguish from an unbalanced vibrations of the steering wheel, right?