Abstract: The invention provides a new automotive tractor unit for use with a new semi-trailer unit to form a rigidly-coupled tractor/semi-trailer combination. The tractor has a long wheel base and has its fifth wheel assembly mounted for extended longitudinal movement between a receiving position at its rear end and a mounting position close to the driver's cab. The tractor and semi-trailer chassis frames cooperate with one another to permit pitching between them while preventing yaw between them. The semi-trailer has a retractable landing gear movable beneath the chassis between a forward position close to the king pin to support the semi-trailer front end in the absence of the tractor, and a rear position close to the running wheels so that the semi-trailer can rest on the tractor and be rigidly coupled thereto. Preferably the tractor wheel base is of length from 6.8 meters (22 feet) to 8.5 meters (28 feet), while the fifth wheel assembly is mounted for a longitudinal movement of from 4.8 meters (16 feet) to 6.