Does driving with windows open waste gas?
It’s hot: is it more efficient to drive with the windows down or air conditioning on?
Do you use more fuel with the windows down or with the air conditioning on? That largely depends on how fast you are going.
As temperatures start to creep up we are once again faced with that eternal dilemma: is it better to wind your windows down or run the air conditioning to keep cool in your car this summer?
Which is more comfortable and which is cheaper are the two big questions, and while the first one is down to personal preference, there have been many studies have been done over the years on the fuel efficiency comparison between windows down or air con, with conflicting results
However, there is one «study» that a lot of people tend to bring up when debating this subject; this is, of course, an episode of the popular American TV series Mythbusters where the hosts compared two «identical» SUVs by filling them with the same amount of petrol and driving them around an oval test track — one with windows down and air con off, one with windows up and air con on — to see which one ran out first.
The SUV with its windows up and air con on ran out first, leading them to claim that air conditioning did indeed use more fuel than winding the windows down, which was clearly true in that particular situation. But there was a lot wrong with that particular situation.
The initial problem with the Mythbusters test is the fact that no two «identical» SUVs are truly identical and a far more accurate test would have been doing the same test with the same vehicle and the same driver twice — once windows down, once air con on — but that doesn’t make for such good television.
Another problem with busting this particular myth is the speed they chose to test it at — the vehicles circulated the oval track at 45mph, or 72kmh, which is awkwardly right in the speed range that more scientific studies have found is the threshold where using air conditioning actually becomes more economical.
Most experts agree that somewhere between 65 and 75kmh is the sweet spot where air conditioning becomes more economical — driving right in the middle of that range with two different «identical» vehicle driven by two different drivers means there are simply way too many variables to get a reliable result.
And, in fact, they got so many complaints about that segment that they did another test later on that found the air con did better in real world open road conditions.
So which way is more economical? Well, it really does depend a lot on your car — how old it is, how well maintained the engine and air conditioning system are and even how aerodynamic it is.
A 2004 study by the Society of Automotive Engineers (SAE) into this very conundrum found that, rather obviously, the more aerodynamic your vehicle is, the more drag dropping the windows creates — a sedan with its windows down showed a decrease in fuel efficiency of 20 per cent overall, while an SUV with its windows down only showed an 8 per cent decrease.
Air conditioning generally adds around a 10 per cent increase in fuel consumption, so clearly you are better off using the air con in a sedan and only marginally worse off using it in an SUV, right?
Well, yes and no — because that previously-mentioned speed cutoff also drastically affects the outcome.
As speed increases the amount of drag on the vehicle also increases. But drag increases exponentially, meaning that when your vehicle is travelling at 100km/h, there is actually around four times more force on the vehicle than when you’re cruising at 50km/h. So even though the vehicle’s speed has only doubled, the drag has actually increased four-fold, drastically affecting your fuel consumption.
So what’s the best option?
At the extreme ends of the spectrum — if you have an old SUV, then windows down all the time is best and, likewise, if you have a brand new super-aerodynamic sedan or sports car, then air con is almost certainly your best option, particularly given the differences in efficiency in both the engines and air conditioning systems of both vehicles.
But for the vast majority of us, a mix of windows down/air con off at town speeds and windows up/air con on at open road speeds is far more likely to provide the most economical option.
As for what you find more comfortable, well, that is up to you, but either way a sensible, measured and logical use of both will almost certainly provide your best balance between fuel economy and comfort.
Is It Bad to Drive with the Windows Open? Your Hearing May Be at Risk
It’s a peaceful early morning around 4:45 AM. Most people are still asleep. You practically have the highway to yourself, moving at a leisurely 55 MPH with the windows down…It seems silly to run the car air conditioner on such a beautiful day. Why would you waste the gas?
But is driving with the windows open bad for hearing?
What science says about driving with the windows down
Don’t believe it’s that loud? British scientists tested everything from a Mazda to an Aston Martin in non-rush hour traffic going 50, 60, and 70 MPH.
They found that regardless of speed or model, when the windows were down, the driver was exposed to a nearly constant 89 decibels of noise.
You get it. Cars are loud. But is this enough to cause permanent hearing loss?
Yes, 85 decibels is enough to cause permanent damage over an 8-hour period. If you’re in heavy traffic, where you’re consistently being exposed to over 100 decibels, permanent damage occurs within 15 minutes.
What factors contribute to the noise when your windows are open?
Your car may seem that loud, but there are a lot of factors that can really increase the decibel noise.
Your engine and vehicle types
Gas engines work by producing a series of rapid explosions in the cylinders. In about 80% of gas-powered cars, engines produce about 85 decibels of sound.
The kind of car you have makes a difference.
Your car may be quieter if you have a hybrid, electric, or small 4-cylinder engine. On the other hand, Ferraris can produce over 100 decibels in first gear.
When you have your windows down, there’s nothing between you and the sound of the engine. In addition, the car vibrates as it moves, producing even more noise.
Wind resistance
Wind resistance adds to the noise mix. The swoosh and whistling sounds of wind moving across your vehicle can significantly increase the noise level even if you have a quieter car.
A study performed by the Henry Ford Hospital Department of Otolaryngology found that cyclists are exposed to 85 decibels of wind resistance at just 15 miles per hour. However, if a cyclist gets up to 60 MPH, the sound is as loud as 120 decibels.
A car has even more wind resistance than a bicycle. So when asking, Is driving with the windows down bad for hearing? The answer is an emphatic “yes.”
Traffic is louder than you think
The sound is magnified every time another vehicle passes you, even on a 4-lane highway. Not only do you hear the engine of the other car, but you also hear their wind resistance.
Noise is worst in these cities.
Try this. Ask your passenger to use a sound meter app on their smartphone while you drive to see how loud it is in your car. Of course, sound is always louder on the driver’s side because you’re closest to oncoming traffic, so keep that in mind while tracking decibels.
The sound meter will show the sound level spikes each time a vehicle passes. Semis and loud motorcycles are among the worst.
A passing vehicle can expose you momentarily to over 100 decibels of sound.
Don’t turn the radio up above the window noise
If you try to turn your radio up so you can hear it over street sounds, then your radio has to be louder than the street. That means the radio may be at 90-100 decibels.
Doppler effect
The Doppler effect happens when something loud is compressing its own sound waves. It’s moving faster than they can. This changes the frequency of the sound you hear.
That’s why you hear a vrooooom sound that seems to change as the car driver gets closer and then moves on past you.
The sound coming out of the car is the same. But the sound waves got compressed.
Very high and very low-frequency sounds can damage your hearing at a lower sound volume. The Doppler effect amplifies sound. It takes sound to its frequency extremes, which is even louder when your windows are open.
Does driving with the windows open save gas?
The short answer is no. Here’s why.
Many people leave the windows down because they think it’s the smart thing to do and that it will save gas. They’re not asking, is driving with the windows down bad for hearing? Instead, they’re often thinking about the practical aspect of things.
They’ve heard that the car air conditioner uses up your gas.
The car AC indeed uses gas. But does it use more gas than the drag caused by rolling the windows down?
The Mythbusters, a popular TV program on the Discovery Channel a few years back, disproved this myth.
They put precisely 5 gallons of gas into two identical SUVs. Then they drove the vehicles at a consistent 45 MPH until both tanks ran out of gas. The SUV with the AC on with the windows up got an extra 15 miles on that 5 gallons of gas.
That’s the equivalent to saving a whole gallon of gas. Wow. Driving with the windows down also wastes a lot of gas. If you do this all the time, you may be wasting 3-4 gallons of gas out of every tank.
The other main reason people drive with their windows down is that it makes them feel young and alive, especially if there’s a great song on the radio. Some people even have convertibles to experience this feeling.
While it’s important to do things you love to do without the weight of fear, it’s essential to understand the risks and reduce them as much as possible.
Let’s look at how to do that next.
How to reduce hearing loss risk while driving
If you love to drive with the windows down or top off, there are some steps you can take to reduce your risk of hearing loss.
If you’re on a long road trip, take regular breaks and alternate between windows up, windows down. Even if it’s far from an 8-hour trip, it’s a smart preventive measure to give your ears a break.
If you have a convertible, rolling the windows up with the top down is enough to get sound below damaging levels.
Get a sound meter app on your smartphone to find out how loud it is in regular traffic. (Never use your phone while driving. Ask a passenger to help.)
Keep the windows up in heavy, moving traffic to avoid prolonged exposure to sound over 100 decibels. Remember, 15 minutes is all it takes to damage your ears when the noise is that loud.
Never try to drown out noise with music. It’s much louder than you think. Wearing headphones or earbuds while driving isn’t just dangerous for you, passengers, and other drivers. It also amplifies the damage caused by drowning out the noise with music by turning it up.
You face the same risks if you’re riding a motorcycle or even a bicycle in traffic. Get a helmet designed to reduce traffic sounds.
Get your hearing tested. If you’ve been driving with the windows down for most of your life, you’ve been very slowly giving yourself hearing loss. Sometimes it’s so gradual that we either don’t realize it or want to admit it.
Is driving with the windows down bad for your health?
The short answer is “yes”. When those windows are down, you’re exposed to engine noise, wind noise, other vehicles, and even your own radio trying to block it out. So take precautions and keep hearing at your best.
IE Questions: Do I Use More Gas In My Car Running The A/C, Or With The Windows Down?
This week, we turn to you, our audience, for our IE Question. Our new focus is all about answering YOUR questions about the energy topics we cover. This time, the age-old: What’s the more fuel efficient way to beat the heat when you’re driving: turning on your air conditioner, or rolling down your windows?
“I’d really like to know what’s the best thing for the gas mileage,” asked divinity graduate student Cathy Jaskey, “because I’m trying to cut down and decrease my carbon footprint.”
You might be thinking, “Didn’t Mythbusters (the Discovery Channel show devoted to investigating myths and folk knowledge using science) do that already?”
The answer is yes, yes they did. They found that it’s more fuel efficient to roll down your windows than to use your air-conditioner.
But not everyone agrees. Public radio’s favorite car experts Tom and Ray Magliozzi, of Car Talk, also addressed this question back in 1990, in their newspaper column, and gave the opposite answer. Ray wrote, “The answer is that you get better mileage by rolling up the windows and using the air conditioner.”
So, who’s right? Well, in the Mythbusters segment, the guys only ran their test with one kind of vehicle – a Ford Expedition SUV – at only one speed: 45 miles per hour. Real driving, however, is a little more complicated.
Steve Iona, physics professor at the University of Denver, says one reason it’s difficult to answer the question is that there are a wide array of variables that affect your gas mileage. One of the most important ones, he says, is the force that slows down your car as it moves through the air. It’s called drag. “In these situations,” he said, “we’re trying to put the car where there is air. So somehow we have to move the air out of the way.” The more drag, the harder your car’s engine has to work to move.
Physics tells us that the most important factor in determining drag is your car’s speed. As you drive faster, drag increases—not proportionally, but exponentially. That means a highway cruising speed of 65 miles per hour works out to a pretty hefty drag force, acting on your car to slow it down.
Iona says speed isn’t the only factor. The shape and surface of the vehicle also affect drag. In general, the less smooth an object’s shape, the larger the drag force. The more you roll down your windows, the less smooth you make the shape of your car. If you were swimming, this would be like starting out with a sleek Olympic uniform, then putting on a t-shirt, and then a sweater, and maybe a fluffy scarf. All this adds up to a drag force that’s greater when you open your windows, and increases exponentially as you drive faster.
What about the other side of the question? What happens if you shut your windows, and turn on the A/C instead?
John Rugh, an engineer at the National Renewable Energy Laboratory (NREL) in Colorado, says running your A/C does hurt your gas mileage, by borrowing some of the energy your car would normally use to move forward. Rugh’s lab has calculated just how much gas that borrowing costs us. According to analysis conducted in 2002, all light vehicles in the United States – that includes SUVs, minivans and sedans – use about 7 billion gallons of gas for air conditioning per year.
That works out to only about five and a half percent of total gasoline use, on average, for the country. Of course, drivers in hotter states probably spend more gas on A/C. We used Census data from 2000 (the same year that the data in NREL’s analysis comes from) to calculate the per-person use of gasoline for each state.
Get the data: CSV | XLS | Google Sheets | Source and notes: Github
The data here are not adjusted for average miles driven in each state, so it may be that people drive more in the states with high per-capita A/C fuel use. It’s also old information – the NREL report is from 2002. John Rugh says they’re in the process of updating the analysis, though he doesn’t yet know when the new report will be released.
Still, 7 billion gallons of gas is significant. But how does it compare to gas used with the windows down? John Rugh says it all depends – once again – on speed. “There could be a crossover,” he said, “where at slow speeds you could drive with the windows down, and if it’s not too hot out, you could be comfortable. And that’s what I do.” But at high speeds, he says, there’s likely to be so much drag that your A/C is more efficient than rolling down your windows.
As with so many energy problems, there are too many variables in play to give a golden number, an ideal speed, for when to switch from your windows to your A/C. So just remember this: [tweetable alt=”Want better gas mileage? Roll down your windows when going slow, use your A/C when going fast.” via “InsideEnergyNow”]Rolling down your windows when you’re going slow, and using your A/C when you’re going fast, is a pretty good bet.[/tweetable]
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Energy is a broad and confusing topic. In this series, Inside Energy reporters de-mystify the wonkiness that dominates so much of the energy conversation, through answering your questions, as well as questions we encounter in the field. What’s your energy head scratcher? Submit it at ask.insideenergy.org, e-mail it to us at Ask@insideenergy.org, or tweet it to @InsideEnergyNow with hashtag #MyEnergyQuestion.
