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Windows down vs AC on: what is more fuel efficient?

by Ajit Menon66,082 Views42 Comments

Does driving with windows rolled down save more fuel than with windows up and air-conditioner on? We help you find the answer



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To roll down your car windows or not – this has been a hotly debated topic, perhaps, ever since car air-conditioners came into existence. Popular belief is that driving with the windows down saves more fuel than rolling them up to turn on the air conditioner. However, an alternate theory posits that driving with windows down hogs as much fuel due to the drag, or the air resistance, it creates on the car. Drag is caused when air enters the car through the windows instead of flowing over it. More power is used in combating this drag, which in turn, burns more fuel. 


So how does one get around this conundrum and what really is the right practice to save fuel? While it’s true that air-conditioners consume a lot of fuel, you’d be surprised to know that a study by US-based Society of Automotive Engineers (SAE) found that driving with the windows up and AC on is more fuel economical than driving with windows down! In fact, their tests showed a 20 percent reduction in mileage with the windows rolled down. With the windows up and AC on, fuel efficiency dropped by just 10 percent. Of course, these results vary depending on the type of car (an SUV experiences higher drag than a small car), how aerodynamic it is, outside temperature, wind speed, etc. 


In general, follow this as a rule of thumb: if you are driving in city and at speeds below 70km/h, roll down the windows and let in the natural air. That’s because at slower speeds, the engine develops less power and hence it has to work harder to generate enough power to run the AC and other components that rely on power from the motor. Besides, when you’re driving slowly, the drag resistance is not strong enough to cause any significant dent to fuel efficiency. At higher speeds, as on highways, turning on the AC would be a better option as the engine would anyway produce enough power to meet the requirements of the AC. Also, the amount of drag on the car will be substantially higher. 


While the jury is still on out on this argument with some experts still insisting on a windows-rolled-down approach, we suggest a combination of the two options to get optimum fuel efficiency. Of course, comfort is another important factor and with our kind of blistering hot, muggy summers, reaching out for that AC button will be a far more natural instinct.


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