AC on or windows down

While the rule of thumb is to keep the windows down on city streets and use air conditioning when taking the highways, it’s been a simmering summer debate since the use of air conditioning in the vehicles.

Drag is one of many causes that play a significant role in the fuel efficiency of any vehicle. It’s common physics that the design needs to be more aerodynamic in order to reduce drag and cars are designed in exactly the same way. So unlike older vehicles, modern cars are designed to be driven with the windows up, so drag can be minimized.

However, driving with the windows open ensures that the air conditioning system also works, which significantly reduces the fuel economy of the vehicle. So, to get some fresh air and save fuel, people tend to close the windows.

While this may save you money in city conditions where the car cannot move faster in bumper-to-bumper traffic, it may cost you more on highways where you drive faster putting more drag on the car.

But does it really matter?

According to a report on 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.

Engineers tested two vehicles, an SUV and a full-size sedan, both with eight-cylinder engines, on a desert track and in a GM wind tunnel.

Engineers found that rolling down the SUV’s windows reduced fuel efficiency by just 8%, in part because the boxy shape of the SUV already created significant drag.

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

“Paradoxically, since 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,” Koerner writes.

“Some engineers have claimed that 45 miles per hour is the break-even point for mid-size cars; others place 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 speed will all play a part in determining your fuel mileage.

In addition to saving your air conditioning for freeway speeds, the U.S. Department of Energy’s fueleconomy.gov recommends the following ways to improve fuel economy in hot weather:

-Do not use AC more than necessary or set the temperature lower than necessary.

– Park in the shade or use an umbrella so the cabin doesn’t get as hot.

– Drive with the windows open for a short time before using the air conditioning. Letting the warm air out of the cabin first will reduce the demand on the air conditioner and help your vehicle cool down more quickly.

– Do not idle with AC power before driving. Switch on the air conditioning after you have started driving or after briefly ventilating the passenger compartment. 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 the range of your vehicle. Also, using a warmer temperature setting for the AC will use less battery power.