About Tyres: tread, size and damage each count separately: which statement is correct?
The tyre requirements (tread, damage, bulging) are only checked at the annual APK inspection; in between it does not matter.
Tyres on one axle must have the same size designation, except when an emergency or spare wheel is used.
As long as tread depth is 1.6 mm or more, it does not matter if the carcass of the tyre is visible somewhere.
When measuring tread depth, you simply count the height of the wear indicators as tread.
Explanation
trap: only-tread-counts. VekaBest p.204 — tyres on one axle must have the same size designation, except when an emergency or spare wheel is used. Enough tread on both tyres alone is therefore not sufficient