The invention concerns a control system for a tractor (10) fitted with two or more axles (21,22) which carry wheels (13,14) having pneumatic tyres and which is set up to pull an implement or trailer. The system having a display means (28) for displaying to a tractor operator tractor (10) performance parameters and has sensors to determine the current pull force applied to the implement or trailer, the current wheel load and the current tractor ground speed. The system also has a control unit which determines and advises the tractor operator via the display means (28) as to the optimum weight of the tractor using a predetermined relationship between pull force and tractor weight and also the required load on each axle to achieve a predetermined optimum axle load ratio.
