It’s a summertime debate that’s been brewing since the invention of the air conditioner. What’s a more efficient use of fuel—driving with the windows down or with the A/C on?

When to Use AC

“The rule of thumb is to keep the windows down while on city streets, then resort to air conditioning when you hit the highway,” says Slate writer Brendan Koerner. “Every car has a speed at which rolled-down windows cause so much drag as to decrease fuel economy more than a switched-on AC. As you might expect, however, that milestone speed varies widely from car to car—and in some cases, it may be well north of posted speed limits.”

In 2004, the Society of Automotive Engineers (SAE) wanted to know how driving with the windows down affected fuel economy and vehicle drag. The engineers tested two vehicles, an SUV and a full-size sedan, both of which featured eight-cylinder engines, on a desert track and in a GM wind tunnel.

Size Does Matter

The engineers found that rolling down the windows in the SUV reduced fuel efficiency by only 8 percent, in part because the SUV’s boxy shape was already creating significant drag. 

But the sleeker and more aerodynamic sedan had a 20 percent reduction in fuel efficiency. The study concluded that the more aerodynamic a vehicle, the greater the loss in fuel efficiency when windows were rolled down at higher speeds.

“Paradoxically, because many fuel-efficient vehicles have low drag coefficients, they may actually experience larger relative increases in drag when the windows are rolled down at high speeds,” writes Koerner. “Some engineers have claimed that 45 miles per hour is the break-even threshold for average-size cars; others put the figure closer to 75 miles per hour.”

Jamie and Adam of Discovery Channel’s MythBusters series might disagree with the SAE. The team drove two SUVs (one with the AC on and the second with the windows down) around Northern California’s Altamont Raceway at 45 miles per hour to see which vehicle would run out of fuel first. In the end, Jamie's air-conditioned SUV stopped first, while Adam's window-cooled car ran for 15 miles more.

In the end, which is better? The answer is: it depends. Variables such as vehicle size and aerodynamics, driving speed, terrain and wind speeds will all play a role in determining your fuel efficiency. To know what’s the best for your vehicle, you might consider testing these theories yourself.

How to Improve Summer Fuel Economy

In addition to saving your AC for highway speeds, recommends the following ways to improve fuel economy in hot weather:

  • Don’t use the AC more than needed or set the temperature lower than needed.
  • Park in the shade or use a sunshade so the cabin doesn't get as hot.
  • Drive with the windows open for a short time before using the AC. Letting hot air out of the cabin first will put less demand on the AC and help your vehicle cool faster.
  • Don't idle with the AC running before driving. Turn the AC on after you begin to drive or after airing out the cabin briefly. Most AC systems will cool the vehicle faster while driving.
  • Read your owner's manual. Most manuals explain how the AC system controls work and how to best use and maintain the AC system.
  • For plug-in hybrids and electric vehicles, pre-cooling the cabin while plugged into the charger can extend your vehicle's range. Also, using a warmer temperature setting for the AC will use less battery power.

Photo credit: nan palmero via photopin cc

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