The paper presents research results of traction abilities of a diagonal tyre 31×15.5-15 on four types of base: concrete, sod, stubble, cultivated soil. Investigation was carried out for two types of air pressure in a tyre: 0.08 and 0.2 MPa and three sizes of a vertical load of a wheel. The objective of the paper was to prove whether the change of air pressure in a tyre influences changes of traction properties of a wheel. Assessment of the impact of air pressure changes was carried out based on measured towing powers, rolling resistance and calculated power and traction efficiency. The research was conducted with the use of the stand for traction investigation aggregated with a farm tractor. The assessment of traction properties proved that a tyre with air pressure 0.2MPa shows better traction abilities - higher values of tractive force on concrete, stubble and cultivated soil, while 0.08 MPa pressure was more advantageous on sod.