What is the maximum permissible dimension X for agricultural truck tractors?

Your answer is correct. The maximum permissible length (dimension X) for agricultural truck tractors in Germany is 12 meters.

This rule ensures that these vehicles, often larger due to attached farm implements, do not exceed a length that would compromise road safety or maneuverability. When tractors and their attachments together approach this length, they must still comply with traffic regulations for safe operation on public roads. Knowing this limit helps you recognize when a tractor is legally allowed on the road and when special permits or warnings might be needed.

In summary, 12 meters is the maximum length allowed for these vehicles to ensure safe and legal driving.