1 by a transmission indicated as a whole by 20 more specifically transmission 20 comprises a first gear reducer 24 a hydraulic joint 25 and a second gear reducer 26 connected at the output to drive shaft 19 according to the present invention an angular speed sensor 28 u2014 e g comprising a pulse generating wheel 29 associated with shaft 18 or any other member rotating at fixed speed with respect to it u2014 detects the rotation speed of auxiliary turbine 13 is connected to a first input 30 of a device 31 for controlling fuel supply and the geometry of turbine 3 and supplies input 30 with a signal i 1 related to the speed of auxiliary turbine 13 a second sensor 34 of conventional type not shown and associated for example with the input shaft of the vehicle transmission to detect the angular speed of the drive shaft hereinafter referred to simply as u201c engine speed u201d is connected to and supplies a second input 35 of device 31 with a signal i 2 0017 fig2 shows a block diagram of device 31 device 31 substantially comprises a first block 36 for calculating the theoretical speed ntcteor of auxiliary turbine 13 on the basis of signal i 2 block 36 is connected to second input 35 substantially comprises a multiplier for multiplying the engine speed value by a constant taking into account the transmission ratio of transmission.