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.