To determine which vehicles must be equipped with a **speedometer**, it is important to consider their speed capacity under normal conditions, that is, on a flat area.

- The **special vehicles** that can reach **60 km/h on a flat area** are mentioned in the texts as examples of vehicles that have specific speed limits on different types of roads. This indicates that these vehicles have the capacity to reach significant speeds and, therefore, need a speedometer to control their speed.

- However, the key is that **all vehicles that can exceed 40 km/h on flat terrain** must be equipped with a speedometer. This includes not only the special vehicles mentioned but also other types of vehicles that can reach or exceed this speed under normal conditions.

Therefore, the correct answer is that **those vehicles that can exceed 40 km/h on flat terrain** must be equipped with a speedometer. This ensures that drivers can monitor and adjust their speed appropriately to comply with the speed limits established on different types of roads.