It's 3 in the afternoon on sunny day, and you get into your car. Do you roll the windows down? Turn on the A/C? The answer's actually clear, and if you've looked into this sort of thing before, it's likely to suprise you.
The best source of information is a study done by the Society of Automotive Engineers (SAE), in 2004. They took a large sedan and an SUV, and tested them at a couple speeds, with the A/C on and off, windows up and down. As most people would expect, there are two big variables in this dilemma - how fast you're going, and how hot it is outside. The SAE study showed conclusively that the temperature of the outside air was the deciding factor - the hotter it was outside your car, the more work the A/C had to do cooling off the air, and the worse your mileage gets.
The suprising finding was that break-even point for outside temperature is just 68°F. This means that if it's more than 68 °F outside, you're better off with the windows down, and the A/C off. Now I can hear the "but what about the drag of having the windows down?" folks starting up, so let's talk about drag.
In a regular sedan, the car does have signifigantly worse aerodynamics with the windows down. Just how much varies across makes and models, but there's a loss in all cases. However, what the SAE report shows is that that loss is smaller that the energy you'd lose with the A/C on. In an SUV, it's simpler. The aerodynamics are so bad on SUVs that opening the windows doesn't make much difference. Another reason that if you don't need an SUV, you can save a lot of money by driving a smaller, more efficient car.
The Fine Print:
Details about the research:
The SAE report tested drag and fuel economy at around 30 and 70mph. They had the A/C units set at a high 75°F, on recirculate - a lot higher than most of us would set our A/C's, especially on a sunny day. There are also a lot of other factors involved: wind speed and direction, engine efficiency, tire temperature, and the shape of your vehicle. It's possible that a very aerodynamic supercar would do better with the A/C on at very high speeds, but odds are, the windows down is going to win out almost all of the time.
Now, unfortunately, the SAE study is the only clear, well-controlled study out there, and it'd sure be nice to have another study to confirm these results. Additionally, some people have read the SAE study in an interesting way, which brings us to...
Treehugger and a few other reports have interpreted the SAE study to say that at high speeds, you should have the windows up and A/C on. However, the data doesn't seem to support this idea. The sticking point seems to be a poorly worded sentence at the end of the report that's tough to understand. Notably, Slate and a few other sources have interpreted the data the same way we have. If someone has an explanation for the windows up, A/C interpretation, please let us know!
Windows down, hands down.
The data's clear, and at higher temperatures, there's just no competition between windows down and A/C. If you can be cool enough driving with your windows down, that's the best way to go.
Now, that's not always possible. If you live in Phoenix and it's 140°F in your car when you get in, and 115° outside, you're likely to have the A/C on. In that case, make sure to have all your windows up, and the air conditioner set to recirculate the air. As your car cools down, it will save a lot of energy by using the already-cool air from the cabin instead of the hot air outside!
So that's this week's Sustainability Showdown. If you've got thoughts, the comment box is below, and if you've got ideas on future topics, let us know!