A load-carrying truck has a fork lift mechanism mounted on a chassis, a pair of front wheels (24, 26) and a single rear wheel (30). At least one front wheel (26) and the rear wheel (30) are driven wheels whose speed is varied relative to one another differently depending on whether the truck is being driven in a forward reverse mode with the front wheels (24, 26) aligned generally parallel to the front-rear axis of the chassis and with steering controlled by steering the rear wheel (30), or a sideways mode with the rear wheel (30) aligned generally perpendicular to the front-rear axis of the chassis and with steering controlled by steering the at least one of the front wheels (26). When the truck is operated in forward/reverse mode and is steered towards the side on which the driven front wheel (26) is located, the relative speed (26) of that wheel is decreased progressively and comes to a stop when the axis of rotation of the rear wheel (30) intersects the front wheel position, and is driven in reverse at increasing speeds as the axis of the rear wheel (30) passes that point of intersection with further increasing steering angle. In the sideways mode of operation when the truck is steered in the direction towards the rear end of the chassis, the relative speed of the rear wheel (30) is decreased progressively and comes to a stop when the axis of rotation of the steered front wheel (26) intersects the rear wheel position, and the rear wheel (30) is driven in reverse at increasing speeds as the axis of the steered front wheel (26) passes said point of intersection with further increasing steering angle.