The objective of the research of tires was to determine the dynamic rolling radius and to apply it to wheel slip calculations with special respect to vertical wheel load and to tire inflation pressure. It is typical of mechanical four-wheel drive tractors that there is a definite additional power in the tractor power chain. This additional power is dependent on the difference between the front wheel and rear wheel peripheral speeds. Further-more, the purpose was to determine the effect of additional slip on four-wheel drive tractors operated without drawbar pull. Experiments were performed on asphalt surfaces and fields. A new measurement method was developed, and a device was constructed for the implementation of three tractor wheel drive operational modes (four-wheel drive, rear-wheel drive and front-wheel drive). As the result of the experiments, a relationship was found to describe the dynamic rolling radius for low-profile radial tires tested on rigid road surfaces. On this basis, the classical slip calculation method was modified. This phenomenon appears only on hard roads and soil surfaces with high adhesion coefficients and only within the low drawbar pull range.