Rims on S6 C4 9x18

Food,

The following problem, I want to drive on my Audi S6 C4 9x18 et 40 with 5mm plates and 255/35, the same back, only et30 without plates.. The rim is Rial Nogaro or something. The report says... front 8 and rear 9, but, vo and rear 255.. Stupid or watt? 255 on an 8??

TÜV says I must have a comparative study.

So, I need an expert report, not a copy of a letter, from a rim, where the expert report says that 9x18 can be mounted around 255. which et don't matter, just about the dimension at the front

Thank you very much