This study determines the effect of tire inflation pressure on the real-time estimation of the rating cone index (RCI) of a soil using wheel sinkage. It is postulated that wheel sinkage, slip, and tire deflection change with tire inflation pressure. The effect of tire inflation pressure on soil strength estimation is verified by using the equation for RCI estimation. Experiments are conducted to measure wheel sinkage, slip, and tire deflection at different tire inflation pressures. The results show an increase in wheel sinkage and slip and a decrease in tire deflection with a change in tire inflation pressure. A statistical analysis of the experimental results is conducted to assess the influence of tire inflation pressure on RCI estimation. The test results show that the calculated the RCI changes with variations in tire inflation pressure. These results help determine the range of tire inflation pressure that indirectly estimates RCI. In addition, a modified equation is proposed to estimate the RCI irrespective of the tire inflation pressure, and its accuracy is verified through calculations. When estimating the RCI through a new equation, constant soil strength can be estimated regardless of the tire inflation pressure. The average error rate of the estimated RCI in each tire inflation pressure was 1.59%.