Does rolling down the car windows and turning off the air conditioner save fuel as you think?

There are two main factors to consider when we approach this question. The first is how the car's air conditioning system works and how much extra fuel the engine has to use to keep running.

The second is air resistance. Air resistance is something that any moving object will encounter when moving through the air at any speed. Most modern cars today are designed with aerodynamics, allowing the car to move through the air with minimal resistance.

Rolling down the windows or turning on the air conditioner while driving will help the engine save more fuel depending on many factors.

However, when the windows are down, air will enter the car cabin, causing more resistance, forcing the car engine to work harder to help your car reach the required speed.

So does drag really affect a car's fuel economy more than air conditioning?

When to use air conditioning to save fuel

In 2004, the Society of Automotive Engineers (SAE) conducted a study at a General Motors wind tunnel and on a desert racetrack. In the wind tunnel, air was forced over the front of the car and also from an angle in front of the car to simulate cross-flow. In the desert, temperature and vehicle speed were included in the study.

Two vehicles were used in the test, one a full-size SUV with an 8.1L V-8 engine and the other a full-size sedan equipped with a 4.6L V-8 engine.

The more aerodynamic a car is, the more fuel it will consume when driving with the windows down.

Overall, both studies found that driving with the windows down had a significant negative impact on fuel efficiency, using more fuel than using the vehicle’s air conditioning. For sedans, driving with the windows down reduced fuel efficiency by 20%, while for SUVs, fuel efficiency was reduced by only 8%.

These differences are an important factor in determining how much the window-down option will affect your vehicle's fuel economy. Research has found that the more aerodynamic a vehicle is, the more fuel-efficient it will be to drive with the windows down. However, with newer, more fuel-efficient vehicles, the difference may be negligible.

SAE recently conducted another test on a new Toyota Corolla at various speeds. They found that with the windows down, the Toyota Corolla had to be traveling at speeds above 130 km/h for aerodynamic drag to make fuel efficiency worse than with the air conditioning on.

SAE also tested a Ford Explorer and found that driving with the windows down while driving was more effective on this SUV at low speeds. But the results also did not make much difference between the two methods when the vehicle was traveling at speeds above 100 km/h.

When to roll down the windows to save fuel

Since no tests have been conducted on a wide range of vehicles, here are some guidelines that you can use. If you drive in the city or at relatively low speeds, your car will use less fuel when the air conditioning is off and the windows are down. This is because driving in this situation creates less aerodynamic drag.

Why? At low speeds, the car engine doesn't need much power, so it won't have to work as hard to power other equipment like the car's air conditioning system compressor.

At low speeds, rolling down the windows while driving will help save fuel.

However, as the speed of the car increases, the air resistance increases. But the resistance does not increase linearly, it increases exponentially. For example, when your car is traveling at 100 km/h, the actual force acting on the car is 4 times greater than when the car is traveling at 50 km/h. So, even though the speed of the car has doubled, the actual resistance has increased 4 times.

If you are looking for a general rule of thumb for when it is best to roll down the windows and turn off the air conditioning, the limit should be around 65 km/h for larger vehicles like SUVs, MPVs and under 50 km/h for small cars like sedans, hatchbacks. If you are driving at 100-120 km/h, you can use the air conditioning while driving without worrying about increased fuel consumption.

But to save the most fuel for your car, you should turn off the air conditioner most of the time. But who can do that on a hot day?

In reality, the benefits or drawbacks of rolling down your windows while driving will depend on a number of factors, from the make of your car, engine displacement, and compressor efficiency. Every car will operate differently, so there is no set rule.

Finally, no matter which method you use, don't forget to keep your tire pressure as recommended by the manufacturer, remove excess items from your car to reduce weight and regularly maintain your drivetrain components, these can bring greater benefits in saving fuel for your car.