It's hot: is it more efficient to drive with the windows down or air conditioning on?

Monday, 24 December 2018

Do you use more fuel with the windows down or with the air conditioning on? That largely depends on how fast you are going.

As temperatures start to creep up we are once again faced with that eternal dilemma: is it better to wind your windows down or run the air conditioning to keep cool in your car this summer?

Which is more comfortable and which is cheaper are the two big questions, and while the first one is down to personal preference, there have been many studies have been done over the years on the fuel efficiency comparison between windows down or air con, with conflicting results

However, there is one 'study' that a lot of people tend to bring up when debating this subject; this is, of course, an episode of the popular American TV series Mythbusters where the hosts compared two 'identical' SUVs by filling them with the same amount of petrol and driving them around an oval test track - one with windows down and air con off, one with windows up and air con on - to see which one ran out first.

Stuck in a long queue on a hot day? Sorry, windows down is definitely the most economical option here. Not necessarily the most comfortable one though.

The SUV with its windows up and air con on ran out first, leading them to claim that air conditioning did indeed use more fuel than winding the windows down, which was clearly true in that particular situation. But there was a lot wrong with that particular situation.

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The initial problem with the Mythbusters test is the fact that no two 'identical' SUVs are truly identical and a far more accurate test would have been doing the same test with the same vehicle and the same driver twice - once windows down, once air con on - but that doesn't make for such good television.

There was no decision to be made in the early days of motoring: there's no air con here. No windows either, for that matter.

Another problem with busting this particular myth is the speed they chose to test it at - the vehicles circulated the oval track at 45mph, or 72kmh, which is awkwardly right in the speed range that more scientific studies have found is the threshold where using air conditioning actually becomes more economical.

Most experts agree that somewhere between 65 and 75kmh is the sweet spot where air conditioning becomes more economical - driving right in the middle of that range with two different 'identical' vehicle driven by two different drivers means there are simply way too many variables to get a reliable result.

And, in fact, they got so many complaints about that segment that they did another test later on that found the air con did better in real world open road conditions.

So which way is more economical? Well, it really does depend a lot on your car - how old it is, how well maintained the engine and air conditioning system are and even how aerodynamic it is.

A 2004 study by the Society of Automotive Engineers (SAE) into this very conundrum found that, rather obviously, the more aerodynamic your vehicle is, the more drag dropping the windows creates - a sedan with its windows down showed a decrease in fuel efficiency of 20 per cent overall, while an SUV with its windows down only showed an 8 per cent decrease.

Air conditioning generally adds around a 10 per cent increase in fuel consumption, so clearly you are better off using the air con in a sedan and only marginally worse off using it in an SUV, right?

Well, yes and no - because that previously-mentioned speed cutoff also drastically affects the outcome.

As speed increases the amount of drag on the vehicle also increases. But drag increases exponentially, meaning that when your vehicle is travelling at 100km/h, there is actually around four times more force on the vehicle than when you're cruising at 50km/h. So even though the vehicle's speed has only doubled, the drag has actually increased four-fold, drastically affecting your fuel consumption.

So what's the best option?

At the extreme ends of the spectrum - if you have an old SUV, then windows down all the time is best and, likewise, if you have a brand new super-aerodynamic sedan or sports car, then air con is almost certainly your best option, particularly given the differences in efficiency in both the engines and air conditioning systems of both vehicles.

But for the vast majority of us, a mix of windows down/air con off at town speeds and windows up/air con on at open road speeds is far more likely to provide the most economical option.

As for what you find more comfortable, well, that is up to you, but either way a sensible, measured and logical use of both will almost certainly provide your best balance between fuel economy and comfort.