Fuel efficiency: AC on or windows down

While driving the thumb rule is to keep the windows down on city streets and resort to air conditioning when you hit the highways, this is a summertime debate that has been brewing since the use of air conditioner in vehicles.

The drag, is amongst many causes which plays important role in fuel efficiency of any vehicle. It is common physics that the design has to be more aerodynamic in order to reduce drag and the cars are designed exactly the same way. So unlike older vehicles, modern cars are designed to be driven with the windows up, so that the drag can be minimized.

However, to drive with the windows up warrants the air-conditioning system to run as well making the fuel economy of the vehicle to take a hit. So, in order to get some fresh air and save fuel, people tend to keep the windows down.

While doing so can save you some bucks in city conditions where the car can’t move faster in the bumper-to-bumper traffic, it can cost you more in highways where you drive faster putting more drag on the car.

But does it really matter?

According to a report published in cenex.com, 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.

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.”

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.

In addition to saving your AC for highway speeds, fueleconomy.gov under the US Department of Energy 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.