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This or that: car air-con or windows down?

How much extra fuel does your car air-con suck up? Is it more efficient to keep your windows down in summer?

By Shell on Nov. 18, 2021

It's one of the car world's great chestnuts – if you want to keep cool but use the minimum amount of fuel, are you better off flicking on your air-conditioning (a/c) or rolling down your windows? Car companies, researchers and even TV's MythBusters have all attempted to find a definitive answer to this question. Here's what's been discovered.

Understanding the science

The Society of Automotive Engineers International (SAE) in the US has conducted two studies into whether a/c or rolled-down windows are more detrimental to fuel consumption involving laboratory and on-road tests of an SUV and sedan at a variety of speeds.

The overarching conclusion of its most recent 2013 study was vehicles consume more fuel with the a/c on than the windows down, echoing the results of an earlier 2004 SAE study (and, incidentally, the MythBusters test results).

So a win for windows down, then? Theoretically, yes, but delve into the details and the picture isn't quite so clear.

Different cars have different a/c systems and how much they impact on fuel consumption differs – car companies don't quantify the numbers, but estimations vary between five and 10 per cent for newer cars, while systems in older cars can be even less efficient. These studies covered just a tiny handful of cars, so it is by no means definitive; even the SAE has stated it would have to test a much broader variety of vehicles to give a clearer answer.

A variety of cars also incur different levels of aerodynamic drag with their windows down, with the SAE concluding that more aerodynamic cars suffer a greater fuel-efficiency impost than boxy, bluff vehicles as speeds rise. In its 2004 test, the sedan's fuel economy dropped up to 20 per cent with its windows open at higher speeds compared to just eight per cent for the SUV.

Putting it into practice

So what does all of this mean? Well, at low speeds – when aerodynamic drag has less of an impact on fuel economy – no a/c and windows down is likely to be more efficient whatever you drive.

Go faster and each car's particular aerodynamics, a/c system characteristics, actual road speed and other factors make a one-size-fits-all recommendation impossible.

If you're driving something sleek, aerodynamic and recent, switching on the a/c is likely to be more fuel-efficient at highway speeds than dropping the windows, but if it's an older, boxy 4WD the opposite could well be the case.

If you have to know, you can test your own car by brimming the tank and driving a varied road loop using one method before re-brimming the tank and noting the kilometres travelled and litres of fuel used. Then repeat the same loop using the other method before brimming the tank again and taking note of the fuel used. Do it a few times and you should get an idea of what's most efficient for your car and driving habits.

Getting the most out of your a/c

How you operate your a/c can make a big difference to its performance and efficiency, as well. Here are a few tips to get the most out of it:

  1. The hotter your car is, the harder the a/c has to work to cool it down and the more fuel you'll use. Parking in the shade if possible is the easiest way to ease the load on your a/c and save fuel.
  2. When you first get in your car on a hot day, open all windows, turn on the a/c, set the temperature to full cold and the fan speed to maximum to purge the cabin of hot air as quickly as possible.
  3. Once the temperature starts to come down, close all windows and switch the a/c to recirculate mode. This will speed up the cool-down process by recirculating ever-colder cabin air rather than the hot stuff outside. 

Disclaimer

Viva Energy Australia Pty Ltd (“Viva Energy”) has compiled the above article for your general information and to use as a general reference. Whilst all reasonable care has been taken by Viva Energy in compiling this article, Viva Energy does not warrant or represent that the information in the article is free from errors or omissions or is suitable for your intended use.