Big wheel problems

[r.ave]

Are you sure it's hitting the bumper and not something else? Do you you have any wear-marks of where its been hitting your car or marks on the tire perhaps?

If the tire size isn't off, maybe the offset of the wheels isn't correct.

Carriage bolts (Slotbouten) would translate to wheel lock nuts.
These would never affect the wheel to rub somewhere in the wheel arch.

Looking at your picture, i'm assuming you are Dutch.
If you want, i have an extra set of wheels that you could testfit, located in Utrecht. 14"x6,5 ET15 and ET25