A tractor-trailer comprising a tractor unit and at least a semitrailer and a control unit configured to receive at least one data item indicating an ambient condition of the ground on which the tractor unit advances; the tractor unit comprises at least a first pair of front wheels, preferably idle and steering wheels, and a second pair of rear wheels, preferably drive wheels, and a braking system of at least one between the first pair of front wheels and the second pair of rear wheels; the semitrailer comprises at least one pair of driven wheels; the tractor unit comprises a device for attaching the semitrailer and a system for lifting the attachment device; the attachment device comprises a rack configured for connecting/disconnecting the semitrailer to/from the tractor unit and means for connecting the rack to a portion of frame of the tractor unit; the tractor unit comprises damper elements positioned at the second pair of rear wheels and a system for lifting the damper elements; the tractor unit comprises a first sensor, positioned at the means for connecting the rack to the portion of frame of the tractor unit, configured for detecting a data item indicating a pulling force of the semitrailer instant by instant; the tractor unit comprises a second sensor configured to detect a data item indicating a pressure of the lifting system of the attachment device; the tractor unit comprises a third sensor configured to detect a data item indicating a pressure of the lifting system of the damper elements; the tractor unit comprises a fourth sensor configured for detecting a data item indicating an angle of steering of at least one wheel of the tractor unit; the control unit comprises a processing unit configured to receive and process the data items (D1; D2; D3) detected by the first sensor, the second sensor and the third sensor and to determine, on the basis of the data items (D1; D2; D3), a relative indication of the load of the semitrailer; the processing unit is configured to establish a reference speed of forward movement on the basis at least of the indication of a load of the semitrailer, data indicating an ambient condition of the ground on which the tractor unit advances and data indicating the steering angle; on the basis of the reference speed established by the processing unit, the control unit is configured for commanding an acceleration or a deceleration at least of a pair of drive wheels (6, 32) of the tractor unit. the tractor unit comprises at least a first pair of front wheels, preferably idle and steering wheels, and a second pair of rear wheels, preferably drive wheels, and a braking system of at least one between the first pair of front wheels and the second pair of rear wheels; the semitrailer comprises at least one pair of driven wheels; the tractor unit comprises a device for attaching the semitrailer and a system for lifting the attachment device; the attachment device comprises a rack configured for connecting/disconnecting the semitrailer to/from the tractor unit and means for connecting the rack to a portion of frame of the tractor unit; the tractor unit comprises damper elements positioned at the second pair of rear wheels and a system for lifting the damper elements; the tractor unit comprises a first sensor, positioned at the means for connecting the rack to the portion of frame of the tractor unit, configured for detecting a data item indicating a pulling force of the semitrailer instant by instant; the tractor unit comprises a second sensor configured to detect a data item indicating a pressure of the lifting system of the attachment device; the tractor unit comprises a third sensor configured to detect a data item indicating a pressure of the lifting system of the damper elements; the tractor unit comprises a fourth sensor configured for detecting a data item indicating an angle of steering of at least one wheel of the tractor unit; the control unit comprises a processing unit configured to receive and process the data items (D1; D2; D3) detected by the first sensor, the second sensor and the third sensor and to determine, on the basis of the data items (D1; D2; D3), a relative indication of the load of the semitrailer; the processing unit is configured to establish a reference speed of forward movement on the basis at least of the indication of a load of the semitrailer, data indicating an ambient condition of the ground on which the tractor unit advances and data indicating the steering angle; on the basis of the reference speed established by the processing unit, the control unit is configured for commanding an acceleration or a deceleration at least of a pair of drive wheels (6, 32) of the tractor unit.