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Analytical studies reveal that for a four-wheel-drive tractor with rigidly coupled drive axles to achieve the optimum tractive performance under a given operating condition, the theoretical speed (the product of angular speed and free rolling radius) of the front tires must be equal to that of the rear tires, or the theoretical speed ratio must be one. This paper presents tractive performance test data obtained using an instrumented four-wheel-drive tractor with seven different sets of tires at various theoretical speed ratios. Field data confirm the analytical findings that when the theoretical speed ratio is equal to one, the slip efficiency and tractive efficiency reach their respective peaks, the fuel efficiency (the ratio of drawbar power to fuel consumed per hour) reaches a maximum, and the overall tractive performance is at an optimum.