Tachograph for a motor vehicle, wherein a control circuit is provided in the tachograph, which control circuit is configured to repeatedly acquire a respective current value for a multiplicity of measured variables during a journey of the motor vehicle and to transmit messages of at least one message type via a communication connection, wherein the respective message type specifies at least one transmission time for the transmission of the respective message of this message type and a selection of the measured variables the respective current value of which should be contained in the message and a format in which the respective current value should be indicated in the message, and wherein the control circuit is configured to generate the respective message of the respective message type for the at least one transmission time, wherein the respectively generated message, in relation to the respective selection of the measured variables, contains their respective current value in the respective format, wherein, in the control circuit, message types are able to be configured at runtime of the tachograph and the control circuit is to this end configured to receive configuration data for at least one user-defined message type via a configuration interface, wherein the control circuit is configured to compile and transmit the messages of the user-defined message type in accordance with the configuration data at the at least one transmission time specified by the user-defined message type and, during the compilation, to ascertain the respective current value of each measured variable selected by the user-defined message type and to convert same into the specified format, wherein the control circuit is configured to keep ready a message-independent value reservoir of the current values in a data memory and to query the at least one value that is required for the message from the value reservoir in order to generate a user-defined message from the value reservoir on the basis of the configuration data, and to operate a conversion module in order to convert the respective current value into the format required for the respective message, which conversion module is configured to carry out at least one predetermined formatting step on the basis of the configuration data. in the control circuit, message types are able to be configured at runtime of the tachograph and the control circuit is to this end configured to receive configuration data for at least one user-defined message type via a configuration interface, wherein the control circuit is configured to compile and transmit the messages of the user-defined message type in accordance with the configuration data at the at least one transmission time specified by the user-defined message type and, during the compilation, to ascertain the respective current value of each measured variable selected by the user-defined message type and to convert same into the specified format, wherein the control circuit is configured to keep ready a message-independent value reservoir of the current values in a data memory and to query the at least one value that is required for the message from the value reservoir in order to generate a user-defined message from the value reservoir on the basis of the configuration data, and to operate a conversion module in order to convert the respective current value into the format required for the respective message, which conversion module is configured to carry out at least one predetermined formatting step on the basis of the configuration data.