From which width onwards must truck tractors be equipped with these warning plates?

Your answer is correct. According to German traffic regulations, truck tractors must be equipped with these warning plates if their width exceeds 2.75 meters.

These warning plates are designed to make wide vehicles more visible to other road users, especially when they extend beyond the common vehicle width. The purpose is to improve safety by alerting other drivers to the oversized width, reducing the risk of collisions.

So, whenever a vehicle, such as a truck tractor, is wider than 2.75 meters, these red and white striped warning plates must be mounted at the widest points of the vehicle. This rule helps ensure that wide vehicles are clearly marked, which is important for the safety of all road users.