Know It All: What’s Better for Fuel Economy — Windows Rolled Down or Air Conditioning?

Everyone struggles to determine the best way to keep cool and conserve gas. Even scientists.

Everybody has that moment when they realize they don’t know about something that they should probably know about. Whether it’s history, language, science, or cultural phenomena, you’ve felt the stinging personal embarrassment of a moment wherein you realize there’s some common knowledge that isn’t so common. Don’t feel bad; nobody knows everything. Nobody, that is, except me and my sidekick, The Internet!

Somewhere in the world, a confused soul begs the question…

What’s better for fuel economy: windows rolled down or air conditioning?

Deep breath, people; this query is not as cut-and-dry as one would hope.

Let’s start officially: the U.S. Department of Energy officially recommends using air conditioning mainly on the highway and going with natural breeze at lower speeds, on surface streets. This is the general rule of thumb adapted by most drivers, as the wind drag becomes unreasonable at speeds upwards of 55 miles per hour. However, the DOE’s website offers no test results to back this up.

A 2005 study by Consumer Reports and Edmunds.com found that, at highway speeds, the difference between using air conditioning and rolling down the windows was negligible and thus challenging both the Department of Energy and common knowledge in one fell swoop. However, rolling down the windows tended to be more fuel-efficient than air conditioning at lower speeds, which confirmed the Department of Energy’s findings.

The Society of Automobile Engineers conducted an extensive “windows down vs. A/C” study in 2004 and ultimately concluded that the “penalty of A/C on at higher ambient [temperature] compared to windows-down is not significantly different for SUV or sedan,” which syncs up with the Consumer Reports/Edmunds findings. Hmm.

Car and Driver found that cranking the air conditioning at 35 miles per hour reduced the car’s fuel efficiency by over 15% but at 55 mph, that number drops to 9.3% and at 75 mph, it dwindles further, 6.8%. Translation: the faster you go, the less impact the air conditioning has on your engine (which will be working harder at high speeds, anyway). In terms of rolling down the windows, they found the average loss of efficiency to be just over 2% at all three speeds, with the fuel efficiency being at its worst at 55 mph. They also tested the effects of an “A/C off, closed windows, open sunroof” methodology, finding it the least detrimental of all three of the cooling solutions (an average decrease of only 1.7%) but still negative, itself.

The Mythbusters tackled the myth twice (once in 2004, once in 2005). The ultimate conclusion was similar to the general rule of thumb – the most effective way to use your car’s gas is to utilize air conditioning mainly on the highway, saving the windows-rolled-down idea for when you’re driving around town.

So, basically, the real answer to this question doesn’t definitively exist. When it comes to comparing air conditioning to rolled-down-windows, it’s a very inconsistent experiment, as cars are difficult to directly compare; any real-world driving scenarios would involve significant variation due to external circumstances like temperature, how cool the driver wants the interior of the car to be, the natural aerodynamics of that car model, wind, tire pressure and temperatures, as well as the car’s mechanical innards.

None of the studies – even those with specific results – support or disprove any of the conventional wisdom that’s already out there and largely, these findings only served to muddy the waters even further by contradicting the findings of others.

We may never get an answer. It makes me sad. I need some air.

Now you know.

Justin Brown is a writer and artist living in Virginia. He channels most of his mind's molten river of creativity into his blog Esteban Was Eaten!. For even more information about him, check out his website.

  • http://air-compressors-for-sale.com/ [email protected] Compressor Sales

    Roll it down. That is, if you got trees lined up. Otherwise global warming is gonna take all over your body faster than you can say “it’s getting hot in here”.

  • Zeph

    There is indeed never going to be a single answer. It depends on how aerodynamic the specific car is with and without windows open; on how many windows are opened by how much; whether the AC unit turns the compressor on and off or runs it all the time that AC is on, and other factors. But reading the sources you link, it looks like windows open is generally lower for most cars even at highway speeds. However the penalty for using AC varies considerably, from minor to significant, depending on other circumstances.

    You got the conclusions of the Hill study published by SAE wrong, tho. You quote a statement saying that there was not much difference in the extra cost of AC between the SUV and the Sedan – meaning that both vehicles when using AC were less efficient than windows open, but by similar amounts. Look at pages 14 and 15 of the linked paper; AC used more Gal/Mi for both vehicles at all tested speeds!

    And you quote Car and Driver as saying that loss of efficiency for open windows was only 2% at all speeds – while it was much higher for AC at all speeds. That does’t support the idea that at high speeds AC becomes more efficient than windows! (We can speculate that the results might be different for a different car, but that’s speculation for now).

  • Zeph

    I do use air conditioning at high speeds or when the temp and humidity are too high for wind. One tip tho: many cars run the AC compressor at full whenever AC is on, and adjust temperature only by mixing in other air (ie: wasting much of the cool air generated by the compressor). So I always keep it on full AC, but I cycle the AC on and off as it gets warm or cool in the car. Some cars may do that for you, but most do not. Also – I start by airing out a hot car with the windows, then to AC with exterior input, then to AC with recirculating air (“max ac”) when I judge the inside air cooler than outside. Combined with manually cycling the AC, this helps average efficiency.