Air Conditioning or Open Windows: Which Uses More Fuel?

Air conditioning is a valuable ally against summer’s highest temperatures, but did you know it can increase a vehicle’s energy use?

To operate, the air-conditioning compressor draws power from the car’s engine - unless you drive an electric vehicle - which places additional load on it and, inevitably, raises fuel consumption.

Fuel consumption can rise by up to 10%, although this depends on several factors. The greatest difference between driving with the air conditioning on or off is typically seen on short journeys.

Why? At the start of a trip, we tend to set the air conditioning to automatic or to maximum, or close to maximum, so that the cabin cools down faster. On a short drive, this means the system’s energy demands remain high throughout.

Why not open the car windows?

When budgets are tight, every saving matters, so opening the car windows may seem like the best option.

However, this comes with a drawback: the vehicle’s aerodynamics. Aerodynamic efficiency is vital for both vehicle stability and fuel consumption, and opening the windows progressively reduces it.

As expected, aerodynamic forces have a greater effect at higher speeds. This is therefore where the largest differences in fuel use appear between driving with the windows open and with them closed.

Still unsure? Picture yourself on the motorway at 120 km/h with the windows down. Not only will the turbulence make it very uncomfortable for your ears, but the extra air resistance will force the engine to work harder to maintain the same speed. More effort means higher consumption.

Open windows and aerodynamic efficiency

It is not easy to find data showing precisely how much fuel consumption increases when driving with the windows open, as aerodynamic efficiency varies considerably from one vehicle to another.

Nevertheless, the limited tests available clearly show that, as a general rule, driving with the windows open at high speeds has a greater impact on consumption than travelling with the windows closed and the air conditioning switched on.

Is there a middle ground?

With this in mind, here are some tips to reduce the impact on consumption while trying to escape the “hell” of a car interior on the hottest days.

Before setting off, start by opening the windows. It is not unusual for the vehicle cabin to be 10–20°C warmer than the outside air, and airing it out through the open windows helps bring its temperature closer to that outdoors.

If your journey is short - both in distance and duration - and you will be travelling at low speeds, it may be best not to use the air conditioning at all. If you do switch it on, you may not even feel its effects before reaching your destination.

For a longer journey, drive for a few minutes with the windows open first. You can then close them and turn on the air conditioning. You may not need to set it as high, either.

As a general rule, when travelling on motorways or dual carriageways, it is more efficient to use the air conditioning than to drive with the windows open.