Data obtained by a tachograph and stored in a memory of the tachograph in a first memory region in a predetermined first data format is processed according to a predetermined processing rule and is stored in a predetermined second data format, which differs from the predetermined first data format, in a second memory region. At least part of the data stored previously in the second memory region is read depending on an inquiry of an external reading unit in relation to the tachograph and is put out via a data interface of the tachograph while maintaining the predetermined second data format.