You are driving your car on a hot summer day. There are two ways to stay cool – close the windows and enjoy the breeze or turn on the air conditioning. Which is better for fuel efficiency? let’s find out
It’s a fairly well-known fact that running the air conditioner in your vehicle has a negative impact on MPG. This applies to both gas and electric vehicles. So rolling down the windows is a better choice for efficiency, right? It is not so easy.
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It’s all about drag
Opening the windows and using the AC both affect MPG, but how they do it is very different. With the windows open, air flows into the car that would normally flow over the vehicle. This creates a resistance known as “drag”.
Cars and other vehicles are designed to be somewhat aerodynamic, which helps minimize drag. When air can easily flow around something, it doesn’t push back as hard. More drag means your vehicle has to work harder to move forward.
Air conditioning is simply an additional component (air compressor) in the vehicle that requires electricity to function. Gas is the primary source of energy in a gas powered vehicle. Everything you do – even charging your phone – uses more of it. Therefore, when the air conditioning is on, your vehicle consumes more petrol.
There are a number of studies on fuel efficiency with windows open versus air conditioning. It boils down to whether the vehicle uses more gas to overcome drag than to run the air conditioning.
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When to open the window
Studies agree that driving with the windows down is more efficient at lower speeds. Essentially we are talking about “city driving”. This means driving on roads that are not highways, stopping at traffic lights, making turns, and so on.
This again has to do with air resistance. When you drive slower, there is less overall drag. Aerodynamics are not that important. Turning off things that use more gas – like air conditioning – will impact MPG more directly.
Because vehicles are so diverse, there is no specific speed limit that works for everyone. studies generally show It’s better for fuel efficiency to open the windows below about 40 MPH. Between 40-75 MPH is more of a gray area.
When to use an air conditioner
As you might have guessed from the previous section, air conditioning is more fuel efficient at higher speeds. This is where drag starts to play a bigger role. It takes more energy to overcome drag than it takes to power the air conditioner.
Again, there is no perfect speed bump that works for every vehicle. A nice rule of thumb is to use AC when you’re on the highway – or speeds over 75 MPH. Anything below that – but over 40 MPH – isn’t as clear-cut. The differences aren’t that pronounced.
A simple rule of thumb
Here’s the short answer — roll down the windows when cruising around town and crank up the air conditioning when you’re on the freeway. If you need specific speeds, open the windows below 40 MPH and use the air conditioning above 75 MPH. The MPG differences in the range between these speeds aren’t that great. Your wallet will thank you.
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