Results of the theoretical assessment with the use of traction research carried out in the field conditions were presented in the article. The assessment concerned the selection issue of the ratio in the power transmission system of a tractor, which met the requirements of universal tractors of 4.0kN class. The maximum power and the engine torque and the ratio of the power transmission system, of the accepted tractor was assumed. Theoretical analysis was carried out for three travelling speeds on four types of ground - such assumptions reflected the universal character of the tractor exploitation. Values of slide and traction force, determined earlier, were used for calculations. The result of the analysis was the power value determination, engine torque and the rotational speed of an engine on particular gears. On the basis of the analysis, which was carried out, the most advantageous ratio in the power transmission system of the tractor including variety of exploitation conditions was indicated. It was proved that exploitation of the tractor on various grounds with the same travelling speed requires the use of different ratios.