A/C ON vs. Windows Down

AC on OR Windows down – What is more fuel efficient?

Although you don't bother asking/searching about this but you still wonder what would be the wiser choice to save a few bucks – windows down with the wind blowing your hair here and there and everywhere or windows up and use the air conditioning instead? Well, as you would expect, it isn't that easy to take a call. Let us look at each scenario separately and that, hopefully, will make things easier to understand.

As is the case in all theories, science has a crucial part to play in these two scenarios.

Aerodynamic Drag: Modern cars are designed to be as aerodynamic as possible, keeping the market it will cater to in check. Still they, and for that matter any object moving through air with a force, face a substantial natural force called aerodynamic drag. In simple English, it is what you feel when you put our hand out of the window while the vehicle is in motion – you get to feel the force of the air on your palm.

Air Compressor: The heart of the Air Conditioner is the air compressor. For your car to keep you in motion and work on the air compressor to keep you cool requires a considerable amount of more fuel.

When driving with the windows down, air passes into the car's cabin whereas it was formerly allowed to flow over the car, causing much more air resistance as compared to closed windows. The best example would be of a parachute. As soon as the skydiver opens the parachute, it cups the air and causes a massive amount of drag, resulting in a drastic reduction in speed with which the skydiver was falling towards the earth. Unlike the parachute however, you definitely don't want a lot of drag on your car because it makes your engine work harder to carry you along at the same speed.

When to use the A/C in a car?
Based on a study conducted by the Society of Automotive Engineers (SAE), driving with the windows up and the A/C on is typically a more fuel-efficient way to go about. However, there are conditions attached to it.

SAE used two cars for this study. An SUV and a full-sized sedan. They were both driven at speeds of more than 88kmph. While the windows were rolled down, for the sedan, the efficiency was reduced by 20%, while for the SUV the efficiency reduced just 8%. This study concluded that the more aerodynamic the vehicle, the more drag open windows will create.

While driving at those speeds, with the windows rolled down, there is a decrease in efficiency by 20% or more. Although using the A/C reduces the efficiency as well by about 10%.
So when driving at speeds which are higher than the usual town speed, rolling up windows and turning on the A/C is a better bet.

When to roll down the windows?
The simple answer is that at town speeds, driving with the windows down is a better option simply because there is less aerodynamic drag at lower speeds. As the speed of your vehicle increases however, the aerodynamic drag increases exponentially. A simple comparison is that if you increase the car speed from say 40kmph to 80kmph, the aerodynamic drag increases nearly four times.

So, a rule that you should all follow to make your journey a lot more environmentally green and slightly easier on your pocket. Use the A/C with the windows up if you are traveling faster than 65kmph and roll those windows down when traveling at lesser speeds than that.