This study investigates the air leakage ventilation phenomenon in a passenger car and examines its effects on the concentration of carbon dioxide in the cabin. A theoretical general equation (QAL={[Acar(vcar2)m1]2+[Afan(vfan2)m2]2}1/2) is proposed for predicting the air leakage ventilation rate. The validity of the equation is demonstrated by means of real car on-road experiments. The results show that when the car is at rest, the effect of the fan-supplied air speed on the air leakage ventilation rate is more apparent. However, for a moving vehicle, the contribution of the fan-supplied air speed to the cabin indoor air quality reduces, while that of the car speed increases. Applying a curve-fitting technique to the experimental data, it is shown that parameters A_car , m₁, A_fan and m₂ in the proposed theoretical equation have values of 1.15x10-4, 0.725, 1.6x10-3 and 0.315, respectively, for the present experimental car (a Mitsubishi Galant). In general, the results obtained in this study suggest that a fractional fresh air ventilation mode should be employed to guarantee the ASHRAE standard for the minimum fresh air requirement of 2.5 l/s per individual at low driving speeds.