signal to the other input of the differential amplifier 3 two leads 6 and 7 simultaneously feed the electrical signals respectively representing the engine speed and the transmission speed to a minimum selector 8 an active rectifier 9 coupled to the output of the differential amplifier 3 rectifies the normally bipolar information appearing at the output of the differential amplifier 3 so that the output is always positive a command source or transmitter 10 responds to the contact k in the gear shift level to control a circuit switch 11 according to one embodiment of the invention the source 10 is a relay coil which is energized through the contact k when gears are shifted and closes contacts s1 and s2 of the switch 11 the latter drives the clutch of the vehicle a lead 15 connects the output of the active rectifier to an input of a control amplifier 16 a lead 17 connects the other input of the control amplifier 16 to the reference variable or nominal value or rated value memory 12 a lead 18 connects the other input of the control amplifier 16 to a controllable discharge circuit 19 the latter is connected by a lead 20 to the output of the speed indicator 1 and by a lead 21 to the minimum selector 8 a differential circuit 22 has an input connected by a lead 23 to the output of the rectifier 9 an output lead 24 connects the output of the differentiator 22 to an input of the control amplifier 16 a lead 25 connects.