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You're driving your car on a hot summer day. There are two options to stay cool---put the windows down and enjoy the breeze or turn on the air conditioning. Which one is better for fuel efficiency? Let's find out.
It's a pretty well-known fact that running the AC in your vehicle has a negative impact on MPG. This is true for both gas and electric vehicles. So rolling the windows down is the better choice for efficiency, right? It's not that simple.
It's All About Air Resistance
Opening the windows and using the AC both have an impact on MPG, but how they do it is very different. When the windows are open, air that normally would flow over the vehicle goes into the car. This creates resistance known as "drag."
Cars and other vehicles are designed to be somewhat aerodynamic, which helps minimize air resistance. If air can flow easily around something, it doesn't push back on it so much. More drag means your vehicle has to work harder to propel itself forward.
Air conditioning is simply an extra component (air compressor) in the vehicle that needs power to function. Gas is the primary power source in a gas-powered vehicle. Anything you're doing---even charging your phone---uses more of it. Therefore, when the AC is on, your vehicle is using more gas.
There have been a number of studies on fuel efficiency with windows open vs. air conditioning. It boils down to whether the vehicle is using more gas to overcome air resistance than it is to run the air conditioning.
When to Open the Windows
The consensus among the studies is that driving with the windows down is more efficient at slower speeds. Essentially, what we're talking about is "city driving." This means driving on non-highway roads, stopping at lights, making turns, and so on.
This has to do with air resistance again. When you're driving at slower speeds, there's less air resistance overall. Aerodynamics is not as big of a deal. Turning off things that use more gas---like air conditioning---will have a more direct impact on MPG.
Since vehicles are so different, there's not a specific speed cutoff that will work for everyone. Studies generally show it's better for fuel efficiency to open the windows under around 40 MPH. Between 40-75 MPH is more of a gray area.
When to Use Air Conditioning
As you may have guessed from the previous section, air conditioning is more fuel efficient at higher speeds. This is when air resistance starts to play a bigger role. It takes more power to overcome the drag than it does to power the air conditioning.
Again, there's not a perfect speed threshold that works for every vehicle. A nice rule of thumb is to use AC when you're on the highway---or speeds above 75 MPH. Anything under that---but above 40 MPH---is not as clear cut. That's where the differences aren't as pronounced.
A Simple Rule of Thumb
Here's the short answer---roll the windows down when you're driving around the city and crank the AC when you're on the highway. If you need specific speeds, open the windows under 40 MPH and use the AC above 75 MPH. The MPG differences in the range between those speeds aren't as great. Your wallet will be thankful.