It’s always nice when you come across an answer that addresses a question that you’ve wondered about? When I saw that Vox, a relatively new site that says it has “the smartest thinkers, the toughest questions” to “explain” our confusing world to us, was running a post on which uses less fuel, running the A/C or opening the windows, I figured I could put the question to bed. While I did find out about the windows down vs air conditioning thing, I also found out that the smart thinkers over at Vox may not be as smart as they think they are.
The article, by Joseph Stromberg, answered my question in the headline, “Why rolling down your cars’ windows is more fuel efficient than using AC”, perhaps in line with that explaining thing. Stromberg’s primary source is a study published by the Society of Automotive Engineers in 2004, “Affect of Windows Down on Vehicle Fuel Economy as compared to AC load“. My first inkling that Stromberg wasn’t completely in command of his sources was when I checked out the study’s presentation. Vox’s writer said “In 2004,the Society of Automobile Engineers tested a full-size V8 sedan and SUV on a desert track and in a wind tunnel, with outside air temperatures around 86°F.” While the SAE indeed published the study, which bears an SAE logo on the title page, on that same page there is a GM logo. Most of the research that the SAE publishes is done by members for their employers, the SAE functions as a clearinghouse to disseminate the information to other engineers. If that logo didn’t make the study’s origin at GM obvious enough, it says that wind tunnel tests were done at GM’s aerodynamic wind tunnel facility according to GM’s internal program and that the road tests were done at GM’s desert proving grounds. A quick search on the lead author, William Hill, also shows that he’s an HVAC engineer at GM.
Okay, so maybe the guy doesn’t cover the automotive beat much and isn’t familiar with the SAE. Well, then there’s the question as to whether the Vox post accurately conveys what it says in the study. Stromberg writes, concerning the fact that running the A/C consumed more fuel than driving with the windows down (at least when the ambient temperature exceeded the A/C settings by at least 15-20 deg F), “The difference was very small for the sedan, and the gap did close even further at high speeds, but it didn’t disappear. For the SUV, the difference was much bigger, and actually increased at high speeds.” While the graphs for the two vehicle types indeed showed those differences between the sedan and SUV, the authors of the study concluded that the differences weren’t significant, not “much higher” as Vox has it: “Penalty of AC ON at higher ambient as compared to windows down is not significantly different for SUV or Sedan [5-10%]“.
Finally, there’s Stromberg’s suggestion, and I can’t tell if he’s being serious or not, that if you really want to save fuel, you should emulate what some hypermilers do. Since it’s true that cars and SUVs are more aerodynamic with the windows closed, some hypermilers will keep the windows up at all times, even in broiling heat, and stay cool by wearing vests with pockets for ice packets. He even links to places where you can buy them. Now using ice to stay cool is not unheard of in the automotive world. I’ve seen ice cubes poured into the racing suits of NASCAR drivers and many race cars today have cooling systems for the drivers, so maybe it’s not such a silly idea. Something the author says about how that ice is made, though, makes me believe that he doesn’t quite understand how energy works.
Some of the best hypermilers wear ice vests to stay cool without windows or AC. They’re not cheap, but you can get a good ice vest for a few hundred dollars, and it’ll pay for itself over the course of a few years of saving gas.
Instead of draining your fuel efficiency, its cooling power is drawn from the freezer you already have running in your house. So if minimizing gas use is your thing, forget about this silly debate, and slip on a vest filled with little ice packs that normal people use to keep food cool at a picnic.
Stromberg couches his suggestion in the context of concern over climate change. He seems to understand that running a refrigeration system in your car takes energy and that the bigger the cooling load, i.e. the greater the difference between the ambient temperature outside and the temperature that the car’s A/C is set to, the more fuel it takes to run the A/C compressor. However, at the same time he doesn’t seem to understand that the freezer in your home works on the same principles. While it’s not as silly as suggesting running the A/C off of a hood mounted windmill with a generator or other perpetual motion-like concepts there’s still no such thing as a free lunch and when you put those unfrozen ice packs, at room temperature, into your freezer that actually increases the load on the freezer’s compressor system. It’s pretty basic science. When I asked a scientist friend who does thermodynamic problems for fun if putting a jug of water in a freezer increases the electrical consumption of the compressor, he said, “Of course, that heat has to go somewhere, why would you ask?”, as though it was too simple to ask about. You might be saving fuel in your car, but you’re using more electricity at home. Overall, you might not really be saving any energy, or money (that would depend on how much you pay for fuel and electricity) at all and still end up wearing a cold soggy vest while you’re at it, with possibly no impact on climate change.
Either way, if it’s more than about 78 or 80 degrees outside and I’m driving, I’m going to have the A/C on, often with the temperature at its lowest setting, the fan at its highest setting, recirculating the air (or, as some car companies labeled the switch back in the early 1970s, Desert Air), just as the good Lord intended Schreiber men to do so, unless I’m running low on fuel.
Charts courtesy of the SAE.
Ronnie Schreiber edits Cars In Depth, a realistic perspective on cars & car culture and the original 3D car site. If you found this post worthwhile, you can get a parallax view at Cars In Depth. If the 3D thing freaks you out, don’t worry, all the photo and video players in use at the site have mono options. Thanks for reading – RJS