Aircon debate: When windows down actually saves fuel

A CapeTalk listener’s uncomfortable Uber ride has reignited a familiar motoring debate: is it more fuel-efficient to drive with the air conditioner on, or with the windows open?

The listener told the station they were travelling in an Uber Go on a hot day when the driver refused to switch on the air conditioning, arguing that comfort features were reserved for higher-tier options and that air conditioning wastes fuel.

That sparked a technical deep dive, with engineer and automotive expert Nicol Louw weighing in.

The power cost of comfort

Lowe explained that a vehicle’s air-conditioning system typically uses between one and three kilowatts of power.

“That doesn’t sound like much,” he said, “but you have to compare it to the extra aerodynamic drag caused by driving with your windows open.”

Open windows disrupt airflow and can impose up to a 10% aerodynamic penalty on a car. Crucially, aerodynamic drag increases with speed, and not in a straight line.

“It goes up with the square of the speed,” Louw explained, meaning the faster you drive, the more energy is needed just to push air out of the way.

The break-even speed

Using calculations based on a typical small car, Louw showed that at around 80 to 100km/h, the energy cost of open windows matches the energy used by the air conditioner.

“At about 100km/h, it doesn’t matter if the windows are open or the air con is on; the fuel penalty is roughly the same,” he said.

Below that speed, however, open windows win.

“At slower speeds, especially in city traffic, it’s more fuel-efficient to drive with the windows open and the air con off,” Louw said, even if it’s far less comfortable.

Comfort still matters

Louw stressed that fuel efficiency isn’t the whole story.

“You can’t really compare the comfort levels,” he said. “Air con doesn’t just cool the air, it removes humidity. You’ll never be as comfortable with open windows.”

For passengers who value comfort, the blunt advice was simple: opt for a higher-tier ride.

Why switching the air con off completely can backfire

There’s also a maintenance angle. Louw warned that never using your air conditioner can cause problems down the line.

“It’s actually a good idea to run your air con at least once a week,” he said. The refrigerant also lubricates seals, and leaving the system idle for long periods can lead to failures.

Re-gassing an air-conditioning system typically costs between R600 and R1,000, often including a basic service.

“If you leave it off all the time, the next time you want it, it probably won’t work,” Louw cautioned.

The bottom line

If you’re cruising on the open road, switching on the air conditioner may save fuel. In slow, stop-start city traffic, open windows are cheaper, but far less pleasant.

For more details, listen to Louw using the audio player below: