Turning on the air conditioning when you’re driving around town can lower your car’s fuel efficiency by as much as 25%. That means it’s much better to roll down your windows. But as your car speeds up, open windows add extra drag to your car, which also makes it less efficient. That’s when you should turn on the AC. 

Also, think about taking steps to make your car cooler before you turn on the engine. Get a sunshade for your windshield, or keep your windows cracked open to let air circulate in your parked car.

Megan Hall: Welcome to Possibly, where we take on huge problems like the future of our planet and break them down into small questions with unexpected answers. I’m Megan Hall. 

Today we have a question from Clay Commons. He wants to know: as the weather heats up, is it better to roll your windows down or run the air conditioning? 

We had Harrison Katz and Fatima Husain from our Possibly Team look into this. Welcome, Harrison and Fatima! 

Harrison Katz: Hi, Megan! 

Fatima Husain: Hello! 

Megan Hall: So, Harrison, when it’s hot outside, should I roll down my car windows or turn on the AC? 

Harrison Katz: Well, before we answer that question, let’s take a step back and talk about how each action affects your car’s gas mileage.

Megan Hall: Ok, let’s start with air conditioning… 

Fatima Husain: So, When you turn on your car’s air conditioning, you’re diverting energy away from the engine, which uses more gas.,

Harrison Katz: The amount of extra gas you’re using depends on the vehicle, but a report from the Society of Automotive Engineers found that turning on the air conditioning when you’re driving around town can lower your car’s fuel efficiency by as much as 25%. 

Megan Hall: Woah, that’s a lot! And does my air conditioner use more gas when it’s hotter outside?

Harrison Katz: Absolutely. And actually, the biggest hit is when the car has been sitting baking in the sun.

Fatima Husain: You know how hot it is when you first get in? It takes a lot of power to cool it down.  

Harrison Katz: For short trips, this makes the AC super inefficient, since the AC works harder to cool off the car and then you just turn it off again. 

Megan Hall: Ok, so how does all of this compare to what happens when you roll down your windows?

Harrison Katz: At first glance, it doesn’t seem like simply rolling down your car’s windows would change much. 

Fatima Husain: And in many cases, it doesn’t. When you’re traveling at low speeds, rolling down the windows has a very small effect on your car’s fuel efficiency. But the faster you’re driving, the worse an option it becomes. 

Megan Hall: Why? 

Harrison Katz: It’s because of a force called drag. Remember when you would stick your hand out the window while your parents were driving, and feel the force pushing it back? That’s drag.

Fatima Husain: Drag is basically air resistance – and the faster you go, the more there is.

Harrison Katz: And drag increases exponentially with speed,. That means every time your speed doubles, your drag quadruples. So, the faster you go with the windows down, the more fuel you waste. Plus, it’s noisy!

Fatima Husain: That means, when you’re going fast, like on a highway, it’s better to roll up your windows and turn on the AC. 

Megan Hall: But when you’re driving slowly, it’s better to keep your windows down? 

Harrison Katz: Exactly. Like for small trips, and even on your way to the highway. 

Megan Hall: Is there anything else people can do to stay cool in their cars while using less fuel?

Harrison Katz: Yes, regardless of the type of trip you’re taking, you can take steps to make your car cooler before you even turn on the engine.  

Fatima Husain: You can get a sunshade for your windshield, which will reflect some of that incoming sunlight. And, if the forecast is clear, you can keep a couple of windows slightly cracked open to allow air to circulate in your parked car.

Megan Hall: Great! Thanks, Harrison and Fatima! That’s it for today. For more information, or to ask a question about the way you recycle, use energy, or make any other choice that affects the planet, go to the public’s radio dot org slash possibly. 

Possibly is a co-production of the Institute at Brown for Environment and Society and the Public’s Radio. 

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