Myth-Busting: Lowering the windows uses more fuel than air-con?
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.
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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.