Not sure if it’s urban myth or not but have always been led to believe that car air conditioning when on will reduce miles per gallon. Has anyone on here actually measured it, what sore of reduction does it make to car fuel efficiency?

Depends on the car – my other half used to have a Ford Ka and the air con used to really affect performance & economy.

I haven’t really noticed a difference in economy with my 1.9 TDI Ibiza but that’s a big, torquey engine in a small car.
It might be a couple of percent, but is within the tank to tank variation, I think.

It used to be much more – compressors were on all the time. Nowadays they are a lot smarter and only run the compressor for a bit.

In the Prius and the Passat it’s impossible to tell. The increase in economy due to hot weather far outweighs any decreased caused by AC because we always get better economy on the hottest days.

I suspect Ford of using crappy technology in their older cars because my Dad’s 53 Fiesta used to make this huge wooshing noise with the aircon on at idle – sounded like a Ranger Rover or something – but no car I’ve had did that.

I only use a/c on open roads (do 2-3 30 mile inter-town trips a week). No noticeable mpg hit. Bit I did notice that I can fairly rev the engine when stopped by using the a/c and demister.

read somewhere that an aircon unit takes about 15 bhp so imagine your car with 15 bhp less and the effect on mpg.

mrs rocket has a Fiesta you can actually feel it ‘ease off’ on the m-way when the aircon starts up. Aircon/hills/headwind/passengers none of it makes any difference to the mighty Mondeo.

Not sure if it’s urban myth or not but have always been led to believe that car air conditioning when on will reduce miles per gallon.

It’s not a myth. To run a compressor takes energy and the only source of energy a car has is the petrol or diesel. Any fuel used to run the AC is therefore not available to propel the car and therfore reduces the mpg.

I remember reading that over about 45-50 mph* it’s more fuel efficient to use the Air-Con than have your windows down.

* or summink like that.

In this weather I couldnt give a monkeys how much extra it used if my mpg halved and the AC kept the car cool I would be happy!

Yeti – depends how hot it is.

If it’s 32 degrees outside having the windows down isn’t going to make you comfortable whatever happens 🙂

I’m not sure it does. Certainly nowhere near as much as learning to drive economically, put it that way

I think it use to cost about 1-2 mpg on our diesel Vectra but I don’t think it makes that much if any in our petrol Focus. That said, I rarely use it TBH. And when I do I tend to not leave it on all the time anyway.

True.

And having them down at slow speeds does next to **** all.

Passat doesn’t have an ‘aircon’ button, it has an ‘economy’ button instead. When this is selected it never uses the AC. When it is, it uses it judiciously to maintain the blower air temperature. Along with electric heaters to blow warm air when the car’s cold 🙂

My skoda has the same climate control system. I reckon using economy saves between 5% and 10%

autoexpress did some tests of this a while back, simulated 30mph urban driving…

30.5mpg – Astra’s baseline fuel economy
30.4mpg – Two front windows a third open
30.3mpg – Roof bars and laden roofbox
30.2mpg – Two front windows totally open
29.7mpg – Four tyres down to 20psi
28.0mpg – Air-conditioning on
27.2mpg – Three large adult males in the back seat
24.1mpg – Roof bars and laden roofbox, plus three adults in the rear, a bike carrier with two adult bikes and air-con

Eco driving: The main facts

– Air-conditioning makes a big difference when driving at low speeds, but has very little impact on the motorway.
– Having the windows up or down makes very little difference.
– A modern aerodynamic roofbox barely impacts on fuel used.
– Slowing down on the motorway will boost fuel economy dramatically.

Read more: http://www.autoexpress.co.uk/news/autoexpressnews/229776/the_mpg_mythbusters.html

A modern aerodynamic roofbox barely impacts on fuel used around town.

FTFY.

I reckon using economy saves between 5% and 10%

I’m fairly nerdy about fuel and have not noticed that, but then I’m not doing the same trips all the time in the same conditions so any effect is drowned out by statistical noise. It certainly has less effect than the weather. 10% would be 5mpg, that’d be noticeable even if I just switched it on whilst driving a flat piece of road.

I’ve got a 2007 C220 Mercedes and the MPG is no different with A/C on or off.

Having the windows up or down makes very little difference.

Only going around town, out a roof box on and do the test for the journeys most people use a roof box for (50-70mph) and the effect will be significant.

read somewhere that an aircon unit takes about 15 bhp so imagine your car with 15 bhp less and the effect on mpg

My Smart diesel only has 45 hp to start with, I’m sure I’d notice if I suddenly lost 15 hp 😀

Dunno, never use it. Ripped it out of my other car as it weighs too much.

It does use fuel – it has to or you could run your fridge at home for free.

But driving when feeling all hot normally makes you drive worse (for economy) so being nice and cool and comfy might actually save you fuel by making you drive more relaxed and not mind taking 2 minutes longer by travelling a bit slower. Speed and changing speed is the biggest factor in MPG as drag, and thus power, is a square relationship to speed and braking is energy lost to heat (unless you drive a Prius).

I think on Mythbusters they discovered that there’s hardly any difference between running the AC and having the windows down.

There’s hardly any difference at all on a modern vehicle, the air con pumps are fairly high efficiency and only a small amount of power is required to do the task at hand (as a percentage of cruise power output) so I’d not expect much more than 1mpg difference on a relatively modern mid-sized car. It should be approximately the same fuel use on any vehicle (roughly the same air con requirements per car except maybe some really small cars) so the % may seem larger on super mini type vehicles (i.e. 2 ml extra per mile is a lot more % wise to a car that only uses 50 for motive power over one that uses 200).

Negligible.

http://www.airconditioningforcars.co.uk/ACpage05.htm

http://www.airconditioningforcars.co.uk/ACpage05.htm

Awesome page “I’ve found no useful figures apart from those in trucks (different kettle of fish)), however here’s my useless figures which have no useful info about AC usage” 🙄 (And from a source with a vested interest in increased AC use!)

dunno, the AC isn’t working at the moment*. it’s a sweatbox on wheels.

* booked in for Saturday, earliest I could get somewhere local

I_Ache – Member
In this weather I couldnt give a monkeys how much extra it used if my mpg halved and the AC kept the car cool I would be happy!

In this weather I use it to keep the windows de misted, 12C and pissing of rain all day. 😥

I think on Mythbusters they discovered that there’s hardly any difference between running the AC and having the windows down

Not a very good comparison though because they had a huge truck with most likely a crude A/C compressor and they compared it to driving with all the windows fully open.

But it doesn’t use FUEL. it uses electricity assuming that a “smart” system uses excess electricity after all systems are charged then no noticeable effect.

But it doesn’t use FUEL

Yes it does! Where else does it get the energy from? (Unless its a hybrid and using regenerative braking)

On a similar note, why do cars with air conditioning have rubbish demisters iff the air-con isn’t on?
I know with the air-con they’re good as they dry the air, but turn it off and you’re left with a misty screen. But in an older car with no air-con, they clear pretty quickly.

How so?

The compressor is driven by the crank.

But it doesn’t use FUEL. it uses electricity assuming that a “smart” system uses excess electricity after

never seen an electric aircon compressor as yet. All I have seen are belt driven off the main crank as already said – which is the most efficient method anyhow ( rotation crank > rotation compressor ) as opposed to electic which would be rotation crank > alternator > compressor motor – each with their efficiency losses )

As for the demisters thing ….

As the warm air from outside has moisture in it, when it passes over the very cold aircon evaporator that moisture in the air condenses on the evaporator matrix and fins. So not only has the system taken the heat out of the air, but also the moisture ( which is wou get all the drips under the car ) So the air that gets into the cabin ( hot or cold ) is drier than the air inside the cabin, and so it soaks up moisture in the cabin ( and thus demists )

It doesn’t work with the AC off cause the moisture isn’t taken out of the air before entering the cabin.

Can’t say it makes a big difference to MPG for me but then my car is pretty thirsty anyway (280bhp Subaru Legacy). There’s a noticeable (but small) loss of power when you press/de-press the aircon button though…

Our Galaxy has climate control & the advice from some of the mechanics on the Galaxy onwers club is to leave it on “Auto” at between 21 & 24deg. The reason being the extra cost of the fuel is nothing compared to the cost of replacing the AC components that can fail if its not used regularly. And 36-40mpg isnt bad for a big car.

Best thing my RS4 has is a solar a/c thingy that keeps the a/c going in the summer when I am not in the car, powered off the sunroof. Come back to a pleasantly cool car instead of blistering hot leather.

Best thing my RS4 has is a solar a/c thingy that keeps the a/c going in the summer when I am not in the car, powered off the sunroof. Come back to a pleasantly cool car instead of blistering hot leather.

A very good idea! Wonder if it can be retrofitted…

Do you have to ‘turn it on’, or does it do it simply when it’s hot enough?

DrP

it uses electricity assuming that a “smart” system uses excess electricity after all systems are charged then no noticeable effect.

…and the electricity comes from where exactly? Even if you have regerative braking all the energy that runs everything in a car ultimately comes from the fuel.

Living in Scotland I can say my mpg is very, very, very rarely affected by a/c!

never seen an electric aircon compressor as yet

Priuses have them.

Best thing my RS4 has is a solar a/c thingy that keeps the a/c going in the summer when I am not in the car, powered off the sunroof

I’d be surprised if it powered the actual aircon rather than just the blower. The Prius has that as an option – whole roof covered in solar and it still only powers the fan, even though the compressor is electric as mentioned above.

Do people actually turn the aircon on and off? I just set mine to whatever temperature I want (usually 18 degrees celcius) and forget about it

randomjeremy – Member
Do people actually turn the aircon on and off? I just set mine to whatever temperature I want (usually 18 degrees celcius) and forget about it

That’ll be climate control & not air-con. I guess climate control is just an ‘intelligent’ air-con, though.

If I had ‘just’ air-con I probably wouldn’t have it on all the time – only when needed.

In my car if you stick the climate control on ‘econ’ which turns the air-con bit off and it starts to get toasty, the system just ramps the fans up to try and get some flow through the cabin & match the ambient to the desired temp on the display. This gets a bit blowy/noisy/annoying so it’s better to just leave it on auto & let it do it’s thing.