Vehicle transmission systems

A transmission system for a four wheel drive vehicle including an input shaft connected to a differential mechanism, which has two output shafts, which are connected, in use, to respective pairs of driven wheels of the vehicle, the two output shafts carrying first and second coaxially mounted sun wheels, respectively, of an epicyclic gear system which mesh with first and second sets of planet wheels, respectively, the epicyclic gear system including a third sun wheel, which is mounted coaxially with the first and second sun wheels and is in mesh with a third set of planet wheels, the gear ratios of the first sun wheel with the first set of planet wheels, the second sun wheel with the second set of planet wheels and the third sun wheel with the third set of planet wheels being different, each first planet wheel being connected to respective second and third planet wheels to rotate therewith about a respective common planet shaft, the planet shafts being connected to a common carrier which is rotatably mounted coaxially with the first, second and third sun wheels, the carrier being connected to a first selectively operable speed changing device, the third sun wheel being connected to a second selectively speed changing device, the transmission system further including at least one sensor arranged to produce a signal indicative of an operating parameter of the vehicle or its engine and a controller connected to the sensor and to the two speed changing devices and arranged to operate the speed changing devices in response to the said signal.