Off-road mobile robotics may have important interest in many fields of application such as agriculture or surveillance. In this paper, the control of a fleet of wheeled mobile robots, equiped with RTK-GPS sensors and communicating through WiFi, is investigated. The focus is particularly set on the control of a formation of several robots with respect to a reference trajectory, previously learned or computed off-line. Non-linear exact transformations permit to achieve a laterally and longitudinally decoupled model; from which the control of steering angle and velocity are derived separately in order to ensure the desired formation shape. Since the control of lateral distance to the reference trajectory is based on other works, only the longitudinal control is detailed in this paper. It is based on an adaptive and predictive control algorithm, in order to account for both sliding and actuator delays. The experimental results demonstrate the capabilities of the proposed approach.