Windows down or air conditioning? The most efficient choice depends on speed

The most useful rule of thumb is simple. Up to around 50 km/h, open the windows if the outside air can cool the cabin sufficiently. Between 50 and 80 km/h, the more efficient option depends on the car and how far the windows are open. From roughly 80 km/h upwards, it usually makes sense to close the windows and switch on the air conditioning.

These are not hard-and-fast laboratory thresholds that apply to every car. Rather, 50–80 km/h is a transition zone in which the energy used by the air conditioning begins to compete with the rising aerodynamic penalty of open windows.

Air conditioning carries a bigger penalty at low speeds

The air-conditioning compressor needs energy to run. In an internal-combustion car, that energy ultimately comes from fuel; in an electric car, it comes from the traction battery. The initial cool-down of a cabin that has been baking in the sun places a particularly heavy demand on the system.

In slow urban traffic, the aerodynamic effect of an open window remains relatively small. At these speeds, turbulence around the opening adds little to the powertrain’s workload. The air-conditioning compressor, meanwhile, still requires energy whether the car is travelling at 20 or 50 km/h.

That is why the US Department of Energy recommends opening the windows at lower speeds and using air conditioning on the motorway. It does not specify a single crossover point because vehicle aerodynamics and air-conditioning loads vary from one model to another.

As a practical guide:
Up to 40 km/h, an open window is usually the most efficient option.
Between 40 and 50 km/h, open windows generally retain the advantage, especially on short journeys.
Between 50 and 80 km/h, the result depends on how many windows are open, how far they are lowered and how hard the air conditioning is working.
From 80 km/h, closed windows usually become the more sensible choice.
At 100 km/h and above, use the air conditioning and keep the windows closed.
One open window creates less drag than opening them all

The effect of opening the windows is not simply a matter of open versus closed. The total open area and the position of the openings matter just as much.

One partially open side window disturbs the airflow less than two or four fully lowered windows. The effect does not rise in direct proportion to the number of windows, however. Air can enter through one opening and leave through another, while pressure differences create very different turbulence patterns depending on which windows are open.

An aerodynamics study published by SAE compared 12 window-opening configurations on a hatchback. Every open-window setup increased drag compared with the fully closed car, but the magnitude of the increase depended on the location and combination of the openings.

That leads to the following practical recommendations.

One slightly open front window

At low speeds, this is the lowest-energy option if the airflow keeps the driver sufficiently cool. A window lowered by only a few centimetres creates less drag than one opened fully.

This approach can remain the more efficient choice up to roughly 50–60 km/h. As speed rises, however, noise increases and the airflow may no longer cool the cabin evenly.

Two open front windows

Opening both front windows ventilates the cabin more effectively, but it also increases the open area and disrupts the airflow more severely. In testing at 80–100 km/h, ADAC measured an increase in fuel consumption of around 0.2 l/100 km with windows open on both sides.

Two open front windows are therefore best suited to urban driving and speeds of up to about 50 km/h. Between 50 and 70 km/h, the result depends heavily on the shape of the car and how far the windows are lowered.

All four windows open

Opening all four windows cools a hot cabin quickly, but it is not an efficient solution for a longer journey. The large open area creates a strong crossflow through the cabin, substantial turbulence and greater aerodynamic losses.

Opening every window is most useful at the start of a journey, when it quickly purges hot air from a car that has been standing in the sun. After a few hundred metres, or once the speed reaches around 40–50 km/h, the windows should be closed.

One rear window open

In some cars, opening a single rear window can produce a powerful low-frequency pressure oscillation. The cabin behaves like a resonant chamber, while the airflow passing over the opening causes the familiar phenomenon known as wind buffeting. Cracking a second window can reduce the pressure difference, but it also increases the total open area and aerodynamic drag. Side-window resonance and noise depend on the shape of the body and how far the window is open.

Using only one window is therefore not always quieter or more comfortable, even though it will generally create less drag than driving with several windows fully open.

Aerodynamics takes over as speed rises

Aerodynamic drag rises roughly in proportion to the square of speed. Doubling the speed from 50 to 100 km/h increases the aerodynamic force by roughly a factor of four, assuming the drag coefficient remains unchanged.

The power required to overcome that resistance rises even faster, roughly in proportion to the cube of speed. In theory, doubling the speed means the car needs about eight times as much power to overcome aerodynamic drag.

That is why a fully open window has little effect on energy consumption at 30 km/h, yet can impose a measurable energy penalty at 100 km/h.

ADAC notes that noise and turbulence from open windows become noticeable from around 50 km/h. Another of the organisation’s efficiency guides says the vortices created around an open window begin to increase energy consumption more clearly from approximately 70 km/h.

This supports the practical 50–80 km/h transition zone. At the lower end, open windows may still be more efficient. Towards the upper end, closed windows and air conditioning running at a moderate setting increasingly come out ahead.

Air conditioning uses the most energy at the start of a journey

The additional energy demand from air conditioning is not constant throughout a journey. In a car that has been standing in the sun, the system must cool not only the cabin air but also the seats, dashboard, glass and other heat-soaked surfaces.

Once the cabin reaches the selected temperature, automatic climate control reduces the compressor load. Air conditioning can therefore draw far more power during the first ten minutes, then operate much more efficiently afterwards.

In an ADAC test, the average increase in fuel consumption caused by air conditioning ranged from 10 to 15 per cent. With a fully automatic climate-control system, the increase was around 5–6 per cent. For a car that normally consumes 5 l/100 km, ADAC calculated an additional 1.0 l/100 km in urban driving and around 0.3 l/100 km at 100 km/h.

In more recent efficiency advice, ADAC gives a possible air-conditioning penalty of roughly 0.3–1.5 l/100 km, depending on the car and conditions. That broad range shows how strongly the result is influenced by temperature, solar load, system design and the driver’s chosen settings.

A laboratory test shows why there is no exact threshold

Oak Ridge National Laboratory compared a 2009 Toyota Corolla with a Ford Explorer. In both vehicles, running the air conditioning at maximum cooling load resulted in higher fuel consumption than driving with the windows open at speeds between 64 and 113 km/h.

In the Corolla, the two methods reached parity at approximately 121 km/h, while open windows became more costly than air conditioning at around 129 km/h. The Explorer still used less fuel with its windows open even above 113 km/h.

That does not mean drivers should keep their windows open at 120 km/h. In the test, the air conditioning was operating at maximum load rather than in a normal automatic mode after the cabin had cooled. The vehicles also dated from an era when climate-control systems were not always as efficient as those in newer models.

The test nevertheless shows that air conditioning does not automatically become the lower-consumption option at 40, 60 or even 80 km/h. A practical decision must account not only for energy use but also for noise, cooling performance, journey length and driver concentration.

Which method is most efficient at different speeds?
Up to 40 km/h

The most efficient choice is usually one or two open windows with the air conditioning switched off.

At these speeds, the aerodynamic penalty remains small. Driving with one partially open window generally uses less energy than driving with several fully open windows, although it cools the cabin more slowly.

40–50 km/h

Open windows usually retain the advantage.

One moderately open window usually makes the most sense when the driver is the only occupant. The benefit of opening two or four windows begins to diminish because the larger open area creates more turbulence.

50–60 km/h

One partially open window may still be more efficient than air conditioning.

With several windows fully open, the car is already entering the transition zone. If the cabin needs to be cooled quickly, it is better to close the windows and use the air conditioning.

60–80 km/h

There is no clear winner.

In moderate weather, driving with one window slightly open may still use less energy. Two or four open windows increasingly compromise the car’s aerodynamics. Air conditioning becomes the more sensible option, particularly once the cabin has already cooled down.

Above 80 km/h

The most practical and usually the most efficient choice is to keep the windows closed and run the air conditioning at a moderate setting.

In some cars, the difference in energy use may remain small, and air conditioning at maximum output can still consume more energy than driving with the windows open. Closed windows, however, reduce noise, improve air distribution and preserve stable airflow around the car.

Above 100 km/h

Keep the windows closed.

At these speeds, driving with several windows open makes little sense because of the noise, turbulence and loss of comfort. Once the cabin has cooled, the air conditioning can maintain the temperature with a lighter, more manageable load than during the initial cool-down.

The most efficient approach uses both methods

It makes little sense to begin cooling a sun-baked car immediately with the windows closed and the air conditioning doing all the work. The hottest air should first be allowed to escape.

The most effective sequence is:
Open the doors and windows before setting off.
Begin driving with all the windows open to flush out the hot air.
Close most of the windows after a few hundred metres.
Once speed rises above 50–60 km/h, close the final window.
Switch on the air conditioning and initially use recirculation mode.
After the cabin has cooled, let the automatic climate control maintain the temperature.
Set the temperature to around 21–23 °C rather than selecting the minimum setting.

This allows the hottest cabin air to escape before the air conditioning takes over, while also avoiding the extra drag created by open windows at higher speeds. Both the US Department of Energy and ADAC recommend ventilating a hot cabin before relying on the air conditioning.

Efficiency is about more than the lowest fuel-consumption figure

Judged purely by energy use, open windows can be more efficient than air conditioning even at motorway speeds in some vehicles and under certain test conditions. In real traffic, however, drivers must also consider noise, air quality, visibility and their ability to concentrate.

Driving in an excessively hot cabin is tiring. Noise from open windows can make surrounding traffic harder to hear, while heavy turbulence makes a long motorway journey uncomfortable.

Efficiency therefore means more than achieving the lowest possible fuel-consumption figure. An effective solution must keep the cabin at a tolerable temperature, avoid unnecessary energy use and allow the driver to remain focused.

In town, an open window will usually do the job. On the motorway, air conditioning does it better.