Opening Windows Versus Air Conditioning

March 20th, 2023

Comments Feed

When the weather gets hotter, it’s important to stay cool when you’re driving.  However, these days, it’s important to consider fuel consumption as well and get the most out of what you’ve paid for – and what we’re going to talk about in today’s article applies to electric vehicles as well!

The two best choices for keeping cool inside the car are using the air conditioning system and the old-fashioned method of opening the windows (if you’re over a certain age, you’ll always try to pantomime cranking a handle to indicate opening a car window.). However, you may have heard people tossing around the idea that opening the window is less fuel efficient. Or you’ve heard that using the air conditioning increases fuel consumption. Which of these is true?

It is certainly true that running the air-conditioning puts extra demands on the engine and consumes more energy when it runs (and this is true of internal combustion engines, hybrids and electric vehicles). This means that when you ask the system for some nice icy-cool air to flow through the cabin and keep you fresh rather than hot and bothered, you increase your fuel consumption.

However, opening the windows affects the drag and aerodynamics of your car. When they design them and test them, designers try to get the drag as low as possible, and they study the way that air flows around the vehicle at speed (usually using wind tunnels as well as computer modelling). This is done to reduce the amount of friction affecting the car, because the more friction that needs to be overcome, the more energy will be required, which requires more fuel, etc. etc. All these tests assume that the exterior of the car is rigid. However, when the windows open, all bets are off and the equations go out the window (almost literally). The open window affect the flow of air, which is how opening the windows cools you down, but it also increases turbulence.

The big question is which is worse in terms of fuel efficiency. Sweltering in the heat just isn’t an option – that’s downright dangerous, especially given some of the temperatures reached in some parts of Australia during summer. So what does the fuel-efficiency-minded person do?

The windows versus air conditioning debate has been going on for some time. In fact, the popular TV show Mythbusters had a go at it. They got both guys driving around a track in similar SUVs, one with the windows down and one with the air conditioning on to see which one ran out of fuel first.

The one with the air conditioning did, which looked like that case should be closed, but it’s not as simple as all that. Firstly, the Mythbusters test wasn’t a strictly controlled one. Even two vehicles of the same make and model will perform differently, depending on a range of factors, including the condition of the engine and the inflation of the tyres. Secondly, the two presenters have different builds and probably have different driving styles, simply because they’re different human beings. To be a more rigorous scientific test, the only thing different should have been the choice between air con and windows open. In other words, the test should have been conducted with the same vehicle driven by the same person with exactly the same conditions – which possibly wouldn’t be the case if you only drove the car once with the air con on and windows up, then with the A/C off and the windows down, as the operating temperature of the engine (cold start vs. hot start) also affects the fuel efficiency. Lastly, one test isn’t enough in the world of science – one result could be just a one-off exception. The ideal is to run test after test after test and see what the general tendency is.

It also gets more complex than that. It turns out that the more aerodynamic a vehicle is to start with, the bigger the effect of drag will be. In other words, in a smooth, sleek sedan, the effect of opening the windows will be greater in terms of percentage than opening the windows on a big chunky 4×4.

To cap things off, speed also has an effect. This is because the faster you go, the more air resistance your vehicle encounters, so the drag increases, and they increase exponentially. This means that if you’re driving at 100 km/h, the effects of drag are four times greater than what you experience at 50 km/h.

The problem was put to a team of actual engineers who ran a proper scientifically rigorous test* to solve the problem. They used two vehicles, a 2009 Ford Explorer to represent the big SUVs and a 2009 Toyota Corolla to represent the sedans. They were tested in the lab and on the road at a variety of speeds and at idle. Here’s what they found:

• At 40–70 miles per hour (that’s 64.4–113 km/h), in both vehicles, turning the air conditioning up to the maximum (which is how they ran the tests) used more fuel than opening the windows.
• Above 70 miles per hour (113 km/h), the two cars behaved very differently.
• At 75 mph (121 km/h), in the Toyota Corolla, there was no difference between having the air con on and having the windows down.
• In the Toyota Corolla, at 80 mph and above (that’s 129 km/h – did they test this legally on an actual motorway or did they have their own circuit somewhere?), having the air conditioning on was more fuel efficient than opening the windows.
• In the Ford Explorer, having the windows down continued to be more fuel-efficient than using the air conditioning.

The study also tested the air conditioning at different settings other than full blast, but you have to pay to see those results!

Of course, not all cars are Toyota Corollas and Ford Explorers, and each has its own drag coefficient and intrinsic fuel efficiency. However, a good general rule of thumb is that if you’re travelling around town, windows down is more fuel efficient. In small sporty vehicles, using the air conditioning is best at open road speeds, but having the windows down is more efficient for big chunky ones.

Here, I will have to add that there are some other advantages of using the air conditioning rather than opening the windows. Firstly, if the outside air is already hotter than comfortable, you’ll only feel a small drop in temperature if you open the windows. It might not be enough to drop temperatures of 40° or more to a nice comfortable room temperature of 18°C. However, the air conditioning will really drop the temperature to this ideal level.

The other problem is that it isn’t just air that can get through the window when its open. Having half a swarm of bees going through the window isn’t the best for safe driving. Nor is having a wasp fly through the window a good idea. Worse still are stones flying up. I’m not making this one up. Last summer, when we were towing a caravan with the windows down and had pulled over to let someone pass, a stone flicked up, glanced off the wing mirror and flew through the open rear window and hit my adult daughter in the face.  A freak accident, I know, but I know that from now on, both she and I will be using the air conditioning on the open road.

* Huff, S., West, B., and Thomas, J., “Effects of Air Conditioner Use on Real-World Fuel Economy,” SAE Technical Paper 2013-01-0551, 2013, https://doi.org/10.4271/2013-01-0551.