When it comes to cooling down your car, it turns out there's really no more fuel-efficient way to do so than to roll your windows down. Yes, even on the highway.
It's not exactly a surprise that rolling your windows down when at slow speeds uses less energy than the air conditioner. However, as news site Vox points out, common wisdom has held that when you're driving on a freeway, the drag on your vehicle that rolling your windows down creates eliminates any gains you get from not using your air conditioning. Turns out that's not true. Even at high speeds, rolling your windows down still saves some petrol:
For both vehicles, having the AC on (blue lines) was consumed more gas than having the windows down (green lines) at all speeds tested.
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.
Of course, rolling your windows down isn't always the perfect option (particularly if you live in a hot, humid place). But if you've got a fuel guzzler and you want to scrape a few extra kilometres out of that tank, turn off the A/C and roll the windows down, no matter where you are.