Does anyone know how much engine power the air conditioning uses, and what this corresponds to in decreased MPG ? And is it true that having the window open decreases the mpg by just as much as the air con due to it increasing the drag of the car ? Also, does greater use of the air con make it more likely to breakdown or need regassing or whatever ? Finally, if it`s not quite warm enough to need the air con on full, if the temperature dial is just set to cool (rather than all the way round to cold) does the air con still use just as much power ?

I`ve often pondered on these questions whilst deciding whether to bother using the air con or not !

The aircon should be used no matter what the temp is mines never off it acts as a dehumidifier so if you have it on hot have the ac on

Aircon uses very little extra fuel it will decrease power slightly it is true its less economical to drive with the window open by a lot more than using the aircon

Finally it should be used all the time to prevent the shaft ceasing once a month I put it on max cold for 10 minutes then max hot on cold it oils the system hot dries out the pipes preventing bacteria build up every year I use a can of cleaner on it aircon on full pointed at footwells press recirc button press can button put in footwell shut door within 10 mins its cleaned the vents iut

Probably 1mpg worse with aircon on, windows open at high speed worse than air con on.

The aircon should be used no matter what the temp is mines never off it acts as a dehumidifier so if you have it on hot have the ac on

Aircon uses very little extra fuel it will decrease power slightly it is true its less economical to drive with the window open by a lot more than using the aircon

Finally it should be used all the time to prevent the shaft ceasing once a month I put it on max cold for 10 minutes then max hot on cold it oils the system hot dries out the pipes preventing bacteria build up every year I use a can of cleaner on it aircon on full pointed at footwells press recirc button press can button put in footwell shut door within 10 mins its cleaned the vents iut

Nice one arthur

I only ever have mine on half if on hot but mostly full way on cold but I must start running it full way on hot and then cold for a bout 10 min each way now and again like you say just to keep the pipes and shafts all in good form.

I guess if it was only used in the summer months and not the winter ect it would not be a good thing for the air con system to be laying up. I also noticed that with A/C on in the warm setting it helps demits rear windows faster. :)

The aircon should be used no matter what the temp is mines never off it acts as a dehumidifier so if you have it on hot have the ac on

Aircon uses very little extra fuel it will decrease power slightly it is true its less economical to drive with the window open by a lot more than using the aircon

Finally it should be used all the time to prevent the shaft ceasing once a month I put it on max cold for 10 minutes then max hot on cold it oils the system hot dries out the pipes preventing bacteria build up every year I use a can of cleaner on it aircon on full pointed at footwells press recirc button press can button put in footwell shut door within 10 mins its cleaned the vents iut

What shaft do you mean?

the shaft on the aircon compressor ....lack of use can cause this to cease when that happens its not a case of replace it you need to flush out the aircon system put filters etc in so that no fragments contaminate the new compressor thats on a standard compressor ford now use from the ti vct climate the vss compressor which is a different setup easy to tell what you have the vss pump the fan runs all the time and the compressor it doesnt cut in and cut out like old ones it runs continously and varies the amount of aircon gas needed youre 2012 fiesta will have a vss pump so not that relevant but it does need to be used to oil the shafts inside the compressor

Focus Mk2.5 with the 1.8 petrol engine - about 0.1gal an hour extra when stationary and engine idle. At 70mph maybe 1-2mpg at most. No noticeable difference in the feel of the power. (AC on, blower on 4, radio up loud, lights on full-beam and mobile phones charging does cause a slight lag in power lol)

Focus Mk2.5 with the 1.8 petrol engine - about 0.1gal an hour extra when stationary and engine idle. At 70mph maybe 1-2mpg at most.

No noticeable difference in the feel of the power. (AC on, blower on 4, radio up loud, lights on full-beam and mobile phones charging does cause a slight lag in power lol)

Very interesting. I have to say I`d have thought a 2mpg penalty at 70mph would be pretty significant, I reckon that`s about 5% worse mpg than what one would expect.

Your second point is even more interesting, how much power does the alternator use ? Obviously that`s primarily dependent upon whether it`s under load. I`ll always remember years ago me and a mate taking the p**s out of this car driver who went past us with no lights on in the dark, it went along the lines of maybe he thinks he`s saving money ! ? ! But, of course, he would be, just not very much.....

To be fair, ot was difficult to gauge - one run was east across the Pennines and the other west. The trip west is the one with AC - and it's generally a thirstier run (longer slog up and across).

The point about all of the electrics was that it is a big load on the alternator. Not AS bad in the Focus, but in lower powered cars it does make a noticeable difference

I dont care. If its hot i put it on! My car is fast enough with it on or off and I prefer comfort to a couple of pound saved in my pocket.