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Aircon effect on MPG

Discussion in 'A3/S3/Sportback (8P Chassis)' started by Skittler, Dec 20, 2006.

  1. Skittler
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    Skittler Redlined

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    [Dec 20, 2006]
    Plenty of previous threads on overall MPG on various models (please let's not start another one), but does anyone know how much difference it makes to MPG when the aircon is on/off?

    I don't get great MPG in mine anyway. Sometimes I get really envious of you diesel boys (usually every half an hour or so while I'm handing over another 40+ quid at a petrol station) but it would still be nice to know the answer.

    I suppose a bit of an experiment would work, such as 100+ miles without it then 100+ miles with it on, but maybe someone already knows?
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  3. wilko
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    wilko Top Gear

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    [Dec 20, 2006]
    Doesn't make any difference I don't think. I've never turned it off or not for long enough.
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  4. JasonM
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    JasonM Member

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    [Dec 20, 2006]
    What ever the difference it would not be enough to recompense you if the aircon failed all together.

    Aircon is therefore best left on all them time.
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  5. S3QuattroDrift
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    S3QuattroDrift Drifting With All 4 Wheels

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    [Dec 20, 2006]
    It's hard to be subjective as i get as high as 32mpg or as low 18mpg depending how i drive. I can only drive like Miss:busted_cop: Daisy for a few miles then this red mist comes over me and i must go fast.
    #4
  6. batwad
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    batwad Member

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    [Dec 20, 2006]
    I read somewhere once that at speeds up to 80mph it worked out more efficient to have your windows open than aircon on in summer. Above 80mph the drag created by opened windows had a negative impact on economy greater than that of aircon (which is pretty much constant with speed).

    As for general driving, a figure I've often heard is that aircon takes away 10% of your engine's power output (so my TDi 130 becomes a TDi 117 with aircon on). Personally, I'm suspicious of that as it means that bigger engines have to work harder to drive their aircon compressors which I can't believe. To know for sure you'd have to know the power consumption of the compressor.

    All of which leads me to the conclusion that aircon is far too enjoyable to be turned off. In fact, I'm actually annoyed by people who turn their aircon off to save fuel. If you can't handle the consequences you shouldn't have bought a car with aircon!
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  7. Vertigo1
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    Vertigo1 Well-Known Member

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    [Dec 20, 2006]
    The only time I ever reach for the ECON button is when the range has been reading 0 for the past few miles and I'm desperately looking for a petrol station :)
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  8. Amchlolor
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    Amchlolor Active Member

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    [Dec 20, 2006]
    The only car I've driven where switching on the aircon had a noticeable effect on the engine was the wife's feeble 1.1 Peugeot 206.
    The engine idle would drop,the lights would go dim etc.
    It was like Dr.Frankenstein throwing the switch !
    Every other car hasn't given a fig whether it's on or not,so I can only assume it has no effect on power/MPG.
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  9. Tim Stuart
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    Tim Stuart Member

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    [Dec 20, 2006]
    Christ - I normally brick it when I get down to 15 miles range.

    Perhaps that's because I trusted the OBC in my Alfa and ran out on the M3 2 miles from Fleet Services and had to run along the hard shoulder to fill a jerry can...
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  10. h5djr
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    h5djr Well-Known Member VCDS Map User

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    [Dec 20, 2006]
    The idea than air-con harms fuel constumption may have been true in the past, but the Audi system uses an electric clutch to engage or disengage the compressor according to the temperture set. Hence the compressor is only being used when it is actually needed to provide cold air and free-wheeling the rest of the time so I very much doubt is it has any real effect.

    The first car I ever had with air-con was an Audi 80 1.8E in 1988 and you could tell when the compressor engaged because there was a slight lowering of the revs on tick-over and a very slight jolt when driving at normal speed. Since then all my cars have had air-con and I've never noticed anything and can never tell if the compressor is engaged or not.
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  11. Vertigo1
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    Vertigo1 Well-Known Member

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    [Dec 20, 2006]
    Well I doubt all cars are the same but I've driven the old car over 10 miles with the range reading 0. Wasn't done deliberately as running out of fuel in a modern diesel isn't funny.
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  12. OutLore
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    OutLore VOIP Dude

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    [Dec 20, 2006]
    I got 22 miles out of my A3 from 0..... But I wouldn't recommend it to anyone...


    Doesnt the guage guess at how many miles you have to go by how you have been driving previously? So if you drive like a lunatic then it will read 0 long before it would if you were driving like a funeral director?
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  13. Vertigo1
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    Vertigo1 Well-Known Member

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    [Dec 20, 2006]
    Well yes but it's relative to how much fuel you have left. If you have a gallon left and have been doing 40mpg then you've got 40 miles left whereas if you've been doing 50mpg you have 50 miles. If you only have a litre left then you'd have 9 and 11 miles left respectively. At zero you have zero, regardless of what mpg you've been doing.

    Of course zero doesn't actually mean zero, they'll obviously build in some tolerance so when it says zero you'll still have a good few litres left, plus it'll probably round down and only shows in 5m increments so as soon as you're below 5 it'll show 0.
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  14. nb
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    nb Very poorly A3 at the moment..........

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    [Dec 20, 2006]
    i think it has very little effect on consumption overall, but it draws off some degree of power from somewhere as sure i read that if you are flooring it the aircon will turn off to channel all available power to the transmission.........another handbook hint i think or could have read it somewhere else
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  15. nb
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    nb Very poorly A3 at the moment..........

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    [Dec 20, 2006]
    and a quickie - zero in the tank does last for a tad bit longer. i have squeezed about 5 miles from mine on empty when the services were refuelling and all garages nr high wycombe decide to close at 10.30pm. but i did coast into the garage forecourt and had to do a fred flintstone to get to the pump.......
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  16. batwad
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    batwad Member

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    [Dec 21, 2006]
    The needle on the fuel gauge in my parent's old Mk3 Escort used to go into the red, to the empty mark and then out the other side and you'd still be pottering along fine. My dad used to call it "running on fumes" :)

    This was obviously hilarious when we ran out of petrol in France heading for our ferry home :(
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  17. Vertigo1
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    Vertigo1 Well-Known Member

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    [Dec 21, 2006]
    You'll get away with that in a petrol but if a modern diesel actually runs out of fuel and ends up sucking on air then you'll be looking at a nice bill from the dealer to sort it out.
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  18. confusionhunter
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    confusionhunter Member

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    [Dec 21, 2006]
    I feel a test coming on, I feel it will make a difference, but there is a lot of people that feel its makes no difference... just about to do a long trip so I'll do one way on fill up and one way off....:)
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  19. nb
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    nb Very poorly A3 at the moment..........

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    [Dec 21, 2006]
    ha ha good work mate!!
    #18
  20. Amchlolor
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    Amchlolor Active Member

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    [Dec 21, 2006]
    Is that still the case ?
    I thought modern diesels were self-bleeding ?

    Not that I have any desire to try it out,just wondering.
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  21. Buzz
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    Buzz Full Throttle

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    [Dec 21, 2006]
    Car air con compressors consume about 5 KW (6 HP) of energy when running. This equates to about 2mpg on your average fuel consumption. Obviously the smaller the engine the greater effect. As previously stated the compressor has an electro-magnetic clutch so it won't be running continously and only when the demand is there.
    #20
  22. wilko
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    wilko Top Gear

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    [Dec 21, 2006]
    I have had about 20 miles from mine before filling it up, from DIS reading 0 miles. If you remember on Top Gear ages ago Clarkson managed to get all the way past the red in an A4 or 6 and it was still going, I must admit sometimes I do wonder when the limit will be hit... but I haven't hit it yet.

    And before I start getting abuse - no I don't do it all the time. I have only done it about 3 times, but sometimes I just ignore DIS when it maons about having to fill up cos it's always on!
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  23. Skittler
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    Skittler Redlined

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    [Dec 21, 2006]
    Thanks to Buzz - who actually managed to stick to the point.

    As batwad says, I wouldn't have bought a £29K car with sub-25 MPG consumption if I was really bothered - I just wondered what the answer was!

    Be interesting to see what confusionhunter comes up with...
    #22
  24. benw123
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    benw123 Moderator

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    [Dec 21, 2006]
    I never notice the compressor engage in my 2.0TDI, but I do use the ECON button wherever possible, particular on dry days in winter when there's little humidity.

    The only car on which I ever noticed the compressor start was my Puma 1.7, probably because it had such little torque. With the A/C on I swear it was more sluggish as a result!
    #23
  25. confusionhunter
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    confusionhunter Member

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    [Dec 29, 2006]
    right gents...... from Edinburgh to dover on almost a tank of fuel..... making progress shall we say with a totall fully loaded A3 110 TDI non PD gave me 45.7.... from Dover to Norwich similar driving and conditions with the aircon on all the time returned 43.6.... just to show it wasnt a fluke i did the Norwich return trip back to Edin with it off again, but I havent filled up yet so dont know only got back this morning..... but I will certainly continue to drive with it off all the time unless it gets hot and for window demisting...... anyone else have any test results confirming or condradicting this?
    #24
  26. marriedblonde
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    marriedblonde Moderator

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    [Dec 29, 2006]
    I've driven home most of the way form from the office with the range showing 0 miles (about 40 miles) and still only had to put in 51 litres. So they definately err on the side of caution.

    J.
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  27. RobinA3
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    RobinA3 Well-Known Member

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    [Dec 29, 2006]
    the lowest i have seen on my car is 15miles on the DIS, i don't like to go low low on fuel as it can damage the CAT if you run the tank near to empty.
    #26
  28. A3_3.2_S-Line
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    A3_3.2_S-Line Member

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    [Dec 29, 2006]
    I don't think it will do anything if you are equipped with a 3.2 i think the only way you can get more mpg is to put a spring behind the accelerator peddle so the right foot has more resistance.

    I have struggled to see over 20mpg over the last couple of weeks.

    My local Shell petrol station even gave me a xmas card for being such a good V-Power customer.:jester:
    #27
  29. RobinA3
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    RobinA3 Well-Known Member

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    [Dec 30, 2006]
    LOL!

    i hope you have a V-Power points card!

    i got sent one a few mths ago as i used optimax and now v power ever since it was released

    just got to see what i want to spend my 21000 points on.............
    #28
  30. Ed
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    Ed Member

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    [Dec 30, 2006]
    I noticed tonight that my car idles at about 800rpm with the AC on, then when i turn it off it idles at 500rpm! Its also alot quieter! Weird!
    #29
  31. RobinA3
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    RobinA3 Well-Known Member

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    [Dec 30, 2006]
    the engine will idle higher when the air con is on as the engine has to run the aircon pump when the air con is on as the air con pump clutch is engaged.
    #30

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