Abstract: A transmission system for vehicles has a coupling (1) with a tubular body (12) provided with internal helical teeth (19,20) at each end. An input shaft (22) is rotatably journalled in the body (12) and is provided with helical splines (25) along its length. A sliding gear (27) has internal gear teeth (26) engaged with the splines (25) and external gear teeth (28) to selectively engage the teeth (19,20) in the body to couple the input shaft (22) and body (12) together. When the body (12) rotates faster than the input shaft (22), the sliding gear (27) moves along the splines (25) to de-couple the body (12) and input shaft (22) until the rotational speed of the two is again equalized. The coupling (11) may be provided between the transfer case and differential of a four wheel drive (4WD) vehicle, in a free-wheeling hub assembly (30) or in a differential assembly (60) for small vehicles.