Abstract: The objective of this paper is to compare vertical stiffness of inflation fluid air and nitrogen to increase capacity of tire normal force and then improve vehicle stability. Tire Stiffness is important to provide comfortable ride for passenger and also prevent damage to the working parts. The stiffness of Tata Nano car tire having size P135/70R12 in the radial [or vertical] direction is measured. This information is based on the measurement of single tire at inflation pressure [1.3, 1.7, 2, 2.4 and 2.7bar] and vertical load range [300 400,500,600,700 and 800kg] for different inflation fluid inside the tire. Experimental verification of vertical stiffness is done for P135/70R12 tire by using load-deflection test conducted for different inflation pressure, inflation fluid and load under static condition. To design mathematical model of undamped spring mass system under harmonic force. The minimum vertical amplitude is obtained from theoretical analysis at given load and pressure for different fluid.