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Vertical stiffness was measured for a 260/80R20 radial ply agricultural drive tire using five methods; load-deflection, non-rolling vertical free vibration, non-rolling equilibrium load-deflection, rolling vertical free vibration, and rolling equilibrium load-deflection. Tests were conducted at three inflation pressures (41, 83, and 124 kPa). Non-rolling free vibration resulted in the highest stiffness for all inflation pressures. Load-deflection and non-rolling equilibrium load-deflection results were similar at all inflation pressures. Rolling vertical free vibration and rolling equilibrium load-deflection results were similar at inflation pressures of 83 and 124 kPa. Non-rolling free vibration is not an adequate method for determining vertical tire stiffness when the tire is represented by a spring and viscous damper in parallel. The discrepancy between load-deflection and free vibration test results was attributed to hysteresis in the tire. Tire properties should be measured at the desired forward velocity when the tire is modeled as a spring and viscous damper in parallel.