Tyres are a final output torque of the vehicle before all the forces are distributed to the road surface as a tractive force. When the tyre pressure changed, it had a direct effect on rolling resistance force of the vehicle and reduced the tractive force. Moreover, it had an effect on a rate of fuel consumption and driving comfort. This research had tested the effect of tyre pressure of 25 psi and 45 psi to determine the total resistance force between the vehicle speed from 40 km/hr to 100 km/hr and compare with the reference tyre pressure of 35 psi by using the “coast-down technique”. The results from calculation showed the total resistance force when the vehicle speed was zero from tyre pressure of 25 psi increased by 48.52% from reference value and the total resistance force from tyre pressure of 45 psi decreased by 13.46% from the reference tyre pressure. The different resistance force from tyre pressure of 35 psi with the other 2 values of tyre pressure in the research were called the different rolling resistance force because the vehicle changed only tyre pressure which had no effect on aerodynamics drag. The relationship of the different rolling resistance force along the test speed from tyre pressure of 25 psi and 45 psi decreased while the vehicle speed increased. The relationship can be fitted with a linear equation. The slope from tyre pressure of 25 psi was 0.2358 which was steeper than the slope of 0.0828 from 45 psi tyre pressure. These were the effect of tyre characteristic that made from rubber. When the vehicle speed was low, the frequency of tyre deformation was also low and resulted in high energy loss from hysteresis phenomena. Therefore high rolling resistance force was generated. Considering the effect of tyre under high vehicle speed, the frequency of tyre deformation increased and made the molecule of rubber vibrate more than low vehicle speed. These effects made the tyre stiffer than low frequency and resulted in decreasing the rolling resistance force. In additional, high frequency that caused by high tyre pressure had effected on driving comfort in a real driving condition.
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