The influence of liquid ballast on tractor tire stiffness was determined from static tests that measured tire load, deflection, and inflation pressure. The tire load-deflection data were fitted with a regression model. The model was differentiated to determine the stiffness of the tire at various combinations of deflection, inflation pressure, and liquid fill. Stiffness at the rated deflection was substantially higher when liquid ballast was used. The increase in stiffness was much less when inflation pressure was reduced to offset the hydrostatic head produced by the liquid fill. The remaining increase in stiffness was due to the change in volume inside the tire during deflection and the smaller volume of compressible fluid in the liquid-filled tire. This study indicates that liquid ballast has a negligible influence on static tire stiffness, if the inflation pressure is determined on a loaded tire and is properly adjusted to compensate for hydrostatic head.