Does the AC use more petrol?



The short answer is - yes it does.
But in most cases not much – it all depends on how the car is used. Please don't be fooled by well meaning journalists with only a limited knowledge of the subject.

For example if you get in your car and drive up the motorway for a hundred miles at seventy miles an hour the amount of extra petrol or diesel used to run the AC is negligible. Indeed if you were to turn the AC off and drive instead with a window slightly open to keep cool, you may well find that the increased drag on the car would increase the fuel consumption more than by using the AC button.

On the other hand if your car is mostly used for short journeys there may be a penalty to pay in petrol for the comfort of continuous AC. For example, you get into a stinking hot car and drive 15 minutes to the supermarket. By the time you get there the car is comfortably cool and you leave it in the full sun for an hour while you do the shop. After the hour the car has returned to its stinking hot status and you need to have the AC on full again for the short journey home. If this is the sort of travelling this car has to do all its life with only short journeys then the fuel consumption is definitely going to deteriorate but after all you get comfort in exchange. For a few minutes each day the compressor is going flat out, taking power from the engine and in addition both the internal blowers and the electric condenser fans are working hard to cool the car down, causing the alternator to work hard to power them and loading the engine further. But if this same car is then used for a long journey, after perhaps ten minutes, once the internal temperature is reduced comfortably, the AC throttles itself back – its done the hard work, now it only needs to keep the car cool and so now the fuel consumption returns to a much more acceptable level.

Similarly if on a hot day you drive along and perhaps note that the fuel consumption on your onboard computer shows 34mpg and you turn on the AC and it immediately drops to 28 mpg it would be easy to assume that you were loosing 6 mpg for the AC. Having read the previous few sentences you can now work out that this simplistic assumption is not in fact correct and that within a few minutes the computer will show a gradual rise to near the point at which it started.

I have found little official research on this but in June 2003 the UK Department of Transport sponsored some research into fuel economy on trucks which included the use of AC. This technical evaluation was done by BTAC/IRTE (British Transport Advisory Committee/Institute of Road Transport Engineers) at the MIRA test track at Nuneaton at the highest speed HGV's are able to do with their speed limiters which is 56 mph (90km/h). At this relatively low speed the effects of an open window are nothing like so serious as they would be at 70 mph, but even so the effect on fuel consumption of the AC switched off and the window open was to increase the consumption by 7% - quite a staggering increase for such a moderate speed. With the windows closed and with the AC switched on, to quote the starchy language of the official report, “the consolidated data suggest that air conditioning has a minimal affect on fuel consumption”.

As I have found so little official research on the effect of AC on mpg in cars I have put in a little of my own experience. Our own car is a 2001 VW Golf 1.6 litre, 16 valve petrol engine. We don't keep fuel consumption records as a norm except when we go on holiday but then we do it properly. We fill the tank to the neck and record the odometer reading, then every time we fill up we record the amount of petrol we buy to two decimal places (and for security we also record the odometer reading). Finally at the end of the holiday we refill the tank to the neck and taking the final odometer reading we calculate the mpg. This is the only truly accurate way to obtain genuine mpg figures.

In September 2006 we had ten days in France and Spain driving a total of about 2800 miles. The majority of the time we were driving on autoroutes with the cruise control set to 80 mph. The AC was switched on for virtually the whole holiday. After calculating the mpg it was a gnats whisker off 44 mpg. I wasn't surprised at this as the previous holiday with the same car in 2005 covering about 2000 miles to the Cote d'Azur and back had given 43.88 mpg, running again around 80mph and again with the AC on for virtually the whole time. Now give that a bit of thought - if we had turned the AC off, how many mpg could we have expected with a 1.6 litre petrol engine at 80 mph? Could we have expected 45 mpg, just maybe but certainly no more than this at these speeds.

There is quite a good report on "How Stuff Works" but it is hardly applicable to the UK or even Europe as the vehicles used in the main trial are of a type extremely rare on this side of the Atlantic (one car had an 8.1 litre engine!). Even the type of AC system is uncommon over here. Nevertheless the findings are somewhat similar to those presented here.

A word about fuel consumption figures. These figures I have quoted above are all in miles per gallon (MPG) but it must be remembered that these are for an Imperial gallon as used in the UK, a gallon in the US is slightly smaller (80% of an Imperial gallon) so if these seem rather generous to a reader from the USA then multiply these by 0.8 to obtain figures that are common to your experience. A further complication is the system used in mainland Europe. This is not just a metric equivalent (km per litre) but is almost exactly the reverse, it is the number of litres of fuel required to drive 100 km (litres/100km). This might seem a little alien to us in the UK or USA but is actually quite a good way of expressing how much fuel is required for a trip in your car - in the example above of our own car on these holidays it would have needed about 6,4 litres to cover each 100 km (62.14 miles) of the journey. If you would like to work it out for your own car then use 282.481 divided by the UK mpg figure.

When I have been driving recently I frequently notice that other modern cars, which surely must have AC fitted, are driving with a window partly open. Perhaps the driver is smoking and wishes to get rid of the ash and smoke. Perhaps the driver is on a very short local journey and can manage without the comfort of the AC. But I often wonder how many are actually on a longer journey and are under the mistaken impression that they will save fuel that way.

About 5 years ago I was repairing the AC of a car at Maidstone when the owner and I heard a car return to the house next door. The following is basically the conversation that I overheard as the owner went to speak to his neighbour.

Owner: Hello George, did you have a good holiday?

Neighbour: Oh, Cornwall was lovely, the weather was great but the journey back was horrible. The car was so hot and the kids wouldn't stop moaning.

Owner: If your AC's playing up I've got the aircon man here working on the Landcruiser right now, he might have time to look at your car.

Neighbour: Oh no, its not that - I don't use the AC 'cos it uses too much petrol.

If it wasn't so sad I could have laughed aloud. He had a nice Peugeot 406 Estate and guess which AC compressor this car uses - its a Sanden SD7V16, the same very economical compressor as VW fit to our Golf. He had three kids in the back, the eldest just into his teens so I expect they and his wife were giving him a very hard time. I bet he had been driving with at least one window open and thus was probably using more fuel than if he had turned the AC on.

An economy tip. If your car has dual position or even 4 position Climate Control and if you are driving without any passengers then if you set the drivers temperature to that you require but turn the passenger side to about ambient temperature and set the fan speed on that side to the lowest position then you should make a small saving on fuel.

A very Special Case

I had an interesting email correspondance in January 2012. A gentleman in Sidney, Australia who was a Security Officer and spent many hours in surveillance at a very hot and sticky location was used to remaining in his car with the engine running the AC in order to turn an almost unbearable temperature into something acceptable. He asked if I knew how much fuel he would be using with the engine ticking over just to provide some cooling. I had to say that not only did I not know the answer to this but I could find no research on this either. If the UK had not been right in the middle of winter I might have conducted an experiment with our own car to find this out but with temperatures hovering around 0ºC this would not have provided any useful data until summer. I did suggest a way to give quite accurate information and he very kindly emailed back early in February to give me the results of his test.

Where he conducted the surveillance he was not able to obtain any shade and had to remain in full Australian summer sun. He found that on a day when the outside temperature was 27ºC and the humidity was 90% he sat in his car and was able to gradually reduce the speed of the interior fan from speed 4 to obtain maximum cooling down to speed 3 after 15 minutes and after 55 minutes actually reduced the speed down to 2 as he cooled right off. In his 2011 Toyota Yaris with a 1.5 litre petrol engine he used 0.93 litres of fuel to run the AC for this hour. I would strongly expect that if he needed to run for a further hour then it would probably have taken slightly less fuel, perhaps 0.85 litres or less for the second hour after the car had been adequately cooled. He was happy that his comfort was not too expensive. A word of warning though - do not try to equate this use with normal driving, you will not be using the best part of a litre of petrol per hour to cool the car whilst driving, this was a very special case.

This is not of much interest to most people but may be of interest to Sales Managers in charge of Reps. Ten years ago at lunch time you would see in any layby a Mondeo or a Vectra in the shade of a tree with the windows wide open and a rep inside drinking coffee from a flask and eating his sandwiches whilst writing up his sales notes. The scene changes now-a-days. The car is no longer in the shade, the windows are now tight shut as the engine is running, ticking over to keep the AC on to provide cooling for the driver whilst he dines and writes. Transport Managers have often wrongly blamed the AC for spoiling fuel consumption - they do need to look more closely at how much time the engine is being used with the car going nowhere.

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