Vehicles fuel consumption under real-world conditions can be up to 60% higher than reported by vehicle manufacturers. One of the most influential factors in this discrepancy is the use of Air conditioning (A/C). However, the study of the contribution of A/C to fuel consumption and emissions has been limited to laboratory tests. This study aims to determine A/C impact on the real fuel consumption in light-duty vehicles. For this purpose, light-duty passenger vehicles were monitored under their normal driving conditions in a 5-month campaign. We obtained, second by second, simultaneous measurements of the vehicle's fuel consumption, speed, position, engine speed and load, and the A/C operational state (on/off). The results obtained showed that the added Specific Fuel Consumption (SFC, L/100 km) due to the A/C peaked at low vehicle speeds (< 10 km/h) with values up to 17.6 L/100 km. This increment reduces with vehicle speed, being (on average) 8.2 L/100 km for low speeds (< 50 km/h) and 1.3 L/100 km for higher speeds. However, when observing this increment in terms of fuel consumption rate, those differences remained approximately constant at 0.9 L/h. Results showed that the SFC metric is naturally influenced by the vehicle’s speed, hiding the real effect of the A/C fuel consumption. Therefore, we suggested to report the A/C added fuel consumption using the fuel consumption rate metric, which showed no clear relationship with vehicle speed. Additionally, we found that the A/C added load to the engine forces it to operate in regions of better efficiencies.