I ask, as I turned mine off (well, switched to econ mode) this morning for the first time in a long time and my fuel economy jumped up by nearly 20% (38 to 45).

Was a surprising jump and I’m wondering if there is something wrong with my aircon system that means its pulling too much power.

It’s a pump, driven by the engine usually. It’ll always take power to drive it so it’ll always effect fuel consumption. It used to be about 2mpg on our Vectra but these days I only turn it in maybe twice a year so I’m not sure about the current car. Your difference seems a bit excessive to me. There’s probably other factors influencing the difference too, and a one-off reading off a fuel computer means diddly squat anyway. Give me 20,000 miles of fuel records and I’ll take some notice… 🙂

if the difference was 1-2mpg on/off I’d tend to agree, this was marked and gets me back to the mpg levels that I would expect but haven’t been getting for a couple of months.

Allegedly the aircon uses about 15 bhp so it’s barely noticeable on the mighty Mondeo but it’s like driving into a headwind on lesser vehicles

It does seem a huge difference – might be worth getting checked, as I’m pretty sure it shouldn’t be that much.

Did you notice the car going better? That much drain would affect performance, Shirley? (ianam)

a word of caution.. leaving aircon of for prolonged periods ‘can’ cause leaks due to Seal swell, or lack off. Back to your your pointhtough, 20% seems like a huge amount, i know i might see a difference of 3-4mpg on the 5 series..

didn’t make a massive difference to the way it drives. its a VAG 1.9tdi.

Terribly difficult to measure because in summer the air is less dense anyway and transmission oils warmer so MPG might improve a little.

I can’t see any difference on my diesel Passat.

jambo – on or off made no significant difference on my 105bhp 1.9tdi VAG…

The cost of running aircon is now thought to be less than that of driving with the windows open.

Rachel

jam bo – Member

didn’t make a massive difference to the way it drives. its a VAG 1.9tdi.

I’ve got the 130bhp version of this engine in my car and can’t detect any change in performance or fuel economy if the air-con is turned on or off (via the econ button on the climate control).
I just leave it on all the time now.

Stupid question, but are you sure this sudden change isn’t due to the high winds today?
If it’s having a real 20% effect on fuel economy, I would expect to notice a performance difference between having it on or off.

My Wife’s old Ka had air con and you noticed the performance hit when that was turned on; but that only had a wheezy 56bhp to start with.

Depends on the car.

On the 1.2 Clios we had in French Guiana, we used to call it the Turbo Boost button. Overtake, floor it, press the AC off button and feel the extra power.

On my Leon 1.8t I can see the extra fuel consumption, but I think I calculated that at about 7% once. But it depends on what roads you drive too.

the air con chills air brought in to the car from out side, the eco setting recirculates that chilled air and cools a little.
So on a hot day, ac on until its cool, then switch to eco. On a long trip switch back to ac for a 10 minutes once an hour or so to refresh the air.

fuel economy used to bounce around the 43mpg mark (its a quattro), last month or so its been down to 38-39. hard to get above 40.

I thought I’d turn pretty much every off this morning apart from lights..and hey presto back up to where I’d expect it.

I drive a focus econetic. With the car running stationary, it shows 0.1 gal/hr. With the air con running, it goes to 0.2 gal/hr. It’s low enough to me to leave it on all the time.

Speshpaul – Member

the air con chills air brought in to the car from out side, the eco setting recirculates that chilled air and cools a little.
So on a hot day, ac on until its cool, then switch to eco. On a long trip switch back to ac for a 10 minutes once an hour or so to refresh the air.

Pretty sure the manual for my car says that ‘econ’ turns off the air con completely (I can’t remember the exact wording). It doesn’t re-circulate the air as that’s what the re-circ button is for.

It also says in the manual that on warm days, with ‘econ’ selected the desired set point may not be achievable, as the fresh air coming into the car will be too warm.
And that’s definitely the case; hit ‘econ’ in the summer and the system can’t keep the temp inside the car at it’s set point.

It probably varies between car models/companies. Perhaps newer cars have a more sophisticated system than the one in my 03 plate?

jam bo – Member

fuel economy used to bounce around the 43mpg mark (its a quattro), last month or so its been down to 38-39. hard to get above 40.

I thought I’d turn pretty much every off this morning apart from lights..and hey presto back up to where I’d expect it.

Economy always seems to take a hit over winter; lower temps, wind, rain, winter-spec fuel (apparently) etc.
Mine’s dropped off since some time in Nov or so.

But that does sound a bit weird that the economy has improved this morning as a result of turning all the ‘load’ off.
If it’s repeatable over a week or so, then I would get it looked at.

Fuel economy according to the OBC? How long a trip?

30mile trip on mostly B-roads but one I do all the time. This morning was pretty average in terms of speed/flow.

yes its the onboard computer but ignore the absolute values (which are pretty accurate, i’ve worked it out manually), its more the large step change I’m interested in.

the air con chills air brought in to the car from out side, the eco setting recirculates that chilled air and cools a little.

Not on my cars. Eco means either less A/C or none at all, usually.

Anyway – it used to be that compressors were big inefficient things, and sized pretty big in the American market so they would draw a lot of power. In my ex’s 1.6 Hyundai Accent you could floor the pedal, then turn off the AC and get a big kick in acceleration. It was noticeably more sluggish with AC on.

However nowadays it’s not much of an issue imo. Compressors are a lot better, and it’ll be switched in and out via a clutch so the less hot it is the less drag there is.

On my diesel Passat econ mode makes less difference than the day to day variations so I can’t tell, and the same is true on the Prius with AC off – and that care is really sensitive to things affecting economy.

Well I’d be inclined not to jump to conclusions based on that. Give it a bit more time, and/or double check it manually.

Meh, I reckon it’s bog all. I’ll leave it running all the time.

On fuel consumption, I drove from Wigan to Hull yesterday with the wind behind me, 55mpg on arrival according to the computer.

Coming home, back into the wind, 45mpg. (2.0TDi 140bhp VAG.)

Allegedly the aircon uses about 15 bhp so it’s barely noticeable on the mighty Mondeo but it’s like driving into a headwind on lesser vehicles

15bhp? Not unless you’re driving an entire aircraft hangar on wheels and keeping it at minus eighteen.

Usually (the “average car”) about 3 to 5kw is needed, i.e. around 2 or 3 hp.

IIRC most modern cars will need about 30bhp to cruise at motorway speeds.

On small-engined petrol cars you can hear the change in engine note if you switch the AC on or off on tickover. On a 2 litre diesel you have to listen hard to hear any change.

Lost in the mix in mine, no doubt it contributes to the overall economy but you can’t tell

I’ve noticed quite a considerable difference in idling speed on the mightymightyBongo with the AC on or off. enough that taking your foot off the brake it begins to accelerate quite well on its own.
But then as its a Japanese spec van they have uprated AC compared to European spec vehicles.
Painfully its returning about 22mpg at the moment….

Turning air con off in my car is like hitting the turbo boost button! It also reduces my mpg down to 25mpg from 30mpg as it takes a lot of power out of my 80bhp monster.

you only get 30mpg out an 80HP engine?

whats the point in that…

Fuel is way way cheaper than air con components and the labor required to fix them.

The change in idle speed shouldn’t be noticeable – the engine should keep idling at the same speed, but the ECU is meant to open the idle control valve (on a petrol, on an oil burner it simply injects more fuel) a little to compensate for the increased load. In fact on Marelli and Bosch ECU’d cars there’s a data feed between the AC ECU and the engine ECU so the engine is forewarned that AC has been requested. The engine ECU can then make a decision as to whether to engage the pump based on a pile of parameters such as load, outside temperature, engine health. Most AC units have a peak demand routine, whereby if full throttle is required, the ECU should disengage the AC pump until the power demand reduces.

Modern AC systems generally have a start-up setting which, if yer mean and like to trundle around with it turned off, will fire up the compressor for a few seconds after start-up just to circulate fresh fluid around the system.

Power consumption is <sfa compared to, say, hauling a spare wheel and a boot load of crap around.

I get 30mpg out of 200hp…

OP: sounds like something’s amiss (assuming that’s a real change). In mine the a/c is off (regardless of what button you press) when outside temp is under 4 deg. So, pressing the button makes no difference until about May…

Are you sure it wasn’t just a tail wind…

hot_fiat – Member

The change in idle speed shouldn’t be noticeable – the engine should keep idling at the same speed, but the ECU is meant to open the idle control valve

Not on all cars. Frinstance Ford Focus lists “listen for the drop in engine revs” as a troubleshooting step for the aircon