Another one that every man and their dog will swear is the case, and one way or the other… does rolling down the windows use more fuel than running the air-con?
BACK BEFORE AIR-CONDITIONING became a thing in cars, we used to roll down the windows and, if you’re old enough, like me, you’ll literally remember rolling down your car’s windows by hand. Using a winder.
Even on a 40-degree day it often felt better to have the windows rolled down in the car then be quietly roasted by the air coming through the car’s vents. This air, before air conditioning, was just being sucked in from outside anyway. But, even while air-con wasn’t a thing, vehicle aerodynamics was and so there were plenty of people who would argue long and loud that rolling down the windows of your vehicle affects its aerodynamic profile and increases fuel consumption.
That argument, on the surface, like most good myths, seems to make sense. You have a solid object designed to cut through the wind and then you place holes in it that seem like they’ll catch the air and increase drag. Yep, makes sense to me. Only, I’ve seen the science, and the answer is not cut and dried.
Yes, there’s actual real, wind-tunnel produced science to support this myth busting… there’s also the TV show Mythbusters which I’m told also produced an episode about this. But this article relies on an SAE produced report that looked at the effect on fuel consumption of rolling down the window vs air-con in both sedans and SUVs at speeds of 50, 80, and 120km/h. Who or what is SAE, I hear you ask… it’s an association of more than 120,000 engineers and ‘related’ technical experts in the field of aerospace, automotive and commercial vehicle industries and it conducts tests, runs seminars and produces journals to improve engineering know-how… it also debunks the odd myth along the way.
Before we get to SAE’s study, because there’s lots to unpack, US automotive publication Consumer Reports wrote about this saying, that running the air-con creates load on the engine that’s equal to the aerodynamic drag of driving with the windows down. And, on the surface that supports the argument that it doesn’t matter which way you go to keep the cabin ventilated, but the truth is dependent on the ambient temperature. And, also the type of vehicle you own.
To the science. According to testing by SAE at an ambient temperature of around 16-degrees C there was almost no difference in whether you ran the air-con unit or lowered the windows. Meaning the fuel consumption from engine load or aerodynamic resistance was equal. But, once the temperature increased the aerodynamic drag created by driving with the windows down compared with the engine load of having the windows up and air-con cranked sees the advantage go to running the air-con. And by a big margin too.
But, even that result isn’t cut and dried, with SAE concluding that low-drag vehicles are more affected (by around 20% of rolling down the windows at high ambient temps) than high-drag vehicles like SUVs (8%). SAE also stated that while its testing was rigorous to realise an accurate result requires multiple runs and that to be definitive it would need to test every single vehicle on the planet.
But the conclusion is that as the temperature increases so does the fuel consumption effect of rolling down your windows. So, yes, rolling down your windows will increase your fuel consumption, so will running the air-con or heating, but not to the same degree.
Back with the FJ, it was windows down and quarter vents wide open and the cabin air intake scoop open – no choice, the A/C didn’t work. I don’t recall it being particularly uncomfortable.
But today’s cars, the back windows particularly set up a buffeting which will almost blow out my ear-drums, very uncomfortable – my new PRIUS is particularly bad – as though the windows are not designed to be put down while in motion.
Thanks Alan,thought I was the only one to experience this.
Agreed Alan. When I was young, many cars had ‘teardrop’ rearmost side windows that opened on hinges at the trailing edge, a fixture I found very agreeable. (I’d like this feature as a modern option.) I’ve had experience with a two door Hyundai Getz, which by design or accident, has a fixed rear window, absolutely flush with the body, hence rear airflow is seemingly silent in rear seats. (claustrophobic & light suspension noise aside). As a pilot, I’m always interested in aerodynamics, and this flush fit not only quietens aircraft interiors, but decreases drag, an extreme factor in flight dynamics and most importantly, aviation economics. In cars at town/city speeds, open windows are slight impediments to fuel consumption, but compared with air con., (& windows up), I’d have to defer to others. Aerodynamics do however come into the picture at, say 80 kph and definitely it’s “windows up” at 100 and above for ‘sanity’ and drag reduction. Here I’m pretty certain that A/C trumps open windows when I’m on a long trip on a freeway or at country speeds. Looking at modern cars, I often ponder the lack of ‘flushness’ of glass and other features such as “styling”. I guess with cars, windows have to be practical for all purposes, but I too, find air buffeting annoying and seemingly counter to other technological advancement elsewhere in cars.
It’s been proven drivers that have their window down that are on the road all day have significant hearing loss in the ear next to the window in the long term.
Also having the window down whilst great on a nice cool day at speeds below 70km/h, after that it’s too noisy. Once it gets hot, AC is a much better proposition.
Don’t worry when 48V systems are common AC will not run off the engine at all, so there’ll be no penalty (well almost)
I’m from the windows down and wet towel era………even vacuum wipers.
And I dare say that there’s a need to consider vehicle speed when calculating drag. The extra windows-down drag at 60kmh should be less than windows-down drag at 110kmh. Do we have any experts on open road summer drag? Where’s Priscilla when you need him?
I drive a 2014 mitsubishi outlander, do not know why it is but the vehicle cannot be driven with the rear windows lowered due to the extreme wind noise thus created.Have never had this problem with any other vehicle.
Hi Tom, all cars are like that. To equalise the pressure and stop the ear-drum bashing sound you’ll need to lower a front window to equalise the pressure. – Isaac
Any chance this thread could be updated with electric vehicles such as a Nissan Leaf? I know the theory is the same, but the draw of electricity by the air-con can reduce range by 10km (assuming 50kph). I’m more interested in motorway speeds of 100kph Aircon vs Windows wound down.
Thanks!