My vans have 8" with 245/60 T/A's on the back and the left (on both) tyres just lightly rub over sharp bumps. Both vans are lowered 2". They don't rub on the top edge of the guard, it's at the back.

It all depends on backspace and dish anyway... I have about 2" I could go inboard without rubbing on anything else so I could squeeze 10" wheels in there at a pinch.

The bag of the tyre has an influence too.