# Effect of air conditioner on mileage

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• posted on August 9, 2005, 5:53 pm

Seems I recall reading a few years back that this type of test might be problematical because of the increased air resistance when driving with the windows open. The theory was that mileage would actually be worse without the A/C. Of course, if you can stand the heat with no open windows or A/C...
Tom
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• posted on August 9, 2005, 8:35 pm
The effect of AC on fuel consumption is minimal, I'm thinking it's going to be about 1/4 gallon per mile, give or take a bit.
That "manifold pressure guage" is really a guage that measures the injector timing to calculate the actual fuel demand. I wouldn't use this guage for what you are trying to figure out, I'd use the On Board Computer's CONSUM feature. I used to think the manifold pressure guage was precisely that, but when you think about how it goes to Max Scale when you take you foot off of the gas and put it on the brake, then drops to Min Scale as the car rolls to a stop, then you have to see that it isn't a vacuum guage at all. What is happening is that the computer calculates how much fuel is in demand at that precise moment, then extrapolates out how far you could go on a gallon if nothing else changed. It doesn't know that you are rolling to a stop and therfore there is no way you can go 99 miles without touching the gas pedal, but it if you actually managed to keep going without using the gas pedal, then you could go as far as the needle says on a gallon.
The On Board Computer looks at fuel demand over time and distance, and gives an average. If you could travel a stretch of road at 80 without the AC on, then come back and do the same stretch at the same speed with the AC on, then you could compare the two averages, and there would not be an appreciable difference. There would certainly NOT be a 10mpg difference.

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• posted on August 9, 2005, 9:15 pm
"J Strickland" wrote

If you were consuming gas at this rate, you'd go through a 20 gallon tank in just 80 miles. :)
Cheers,
Pete
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• posted on August 9, 2005, 10:54 pm
If one gets 25 mpg with the AC off, and 24.75 mpg with it on, then the difference in range on a tank of gas would be 3.75 miles, assuming 15 gallon fill up.
I don't know what numbers you were running, but you were not running a quarter gallon per mile.

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• posted on August 9, 2005, 11:26 pm
"J Strickland" wrote

OK, so it's a quarter of 1 mpg, not quarter of gallon per mile. :)
Otherwise, if you use up 1/4 of a gallon per mile, then that means after 4 miles you used up 1 gallon, and after 40 miles you used up 10 gallons, and so on...
Pete

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• posted on August 9, 2005, 11:44 pm

Yes, a quarter of one mile per gallon, which is much different than a quarter of a gallon per mile.
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• posted on August 11, 2005, 5:08 pm
wrote:

I reckon that the aircon reduced my mpg from around 50 to 47 on a regular 60 mile run I do. Oh and that is on a 1 yr and 11 day old 320D.

-- John Perry
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• posted on August 11, 2005, 6:08 pm

I seriously doubt that the AC alone sucked up 3 mpg. I suspect that if you made the same run several times, some with and some without the AC blowing, the average consumption rate would be almost identical - with the variation being closer to .25 mpg. If you get 50 most of the time, then the AC should turn in a number of about 49.75. I think there have to be other factors in the changes you report, long time standing in traffic, mashing the pedals harder, starting the car then going back inside to pee and look for your cell phone, those sorts of things.
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• posted on August 11, 2005, 6:49 pm
"J Strickland" wrote

It's a bit steep, but not impossible, I think. Losing 3 mpg at 50 is not the same as losing 3 mpg at 20, for example. Percentage-wise, going from 50 to 47 mpg, you only worsened your mileage by 6% (still a lot though). Going from 20 to 17 mpg, your mileage worsened by 15% (terrible).
For me it's easier to see this using metric units: Going from 50 to 47 mpg, means he's using only 0.4 liters more per each 100 km. Going from 20 to 17 mpg, means he'd be using 2.5 liters more per each 100 km.
Also, the weaker the engine, the more visible the effect of A/C on mileage will be. Finally, there are so many different factors (wind speed and direction, for example), that I agree, he should have done more runs and averaged out the results to minimize the error.
Regards,
Pete
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• posted on August 12, 2005, 12:15 am
Well, I really appreciate all of the informed commentary that this little posting of mine has generated.
For the record, my analog MPG needle drops at least 10 indicated units when I turn on the AC, and gets them back immediately when it is turned off.
Many have commented that this needle readout is calculated from actual consumption data, but I contend that it is really manifold pressure. At idle, in Park, no motion (such as just being started first thing in the morning) the readout is 'infinite' MPG, i.e. pegged on the high end of the MPG scale. This is exactly the way manifold pressure gauges on airplanes work, it is the only thing in my experience that works this way.
(Car again is 1996 750iL with about 110K miles).
Thanks to all.
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• posted on August 12, 2005, 2:02 pm
Sun God wrote:

Your contention is very nice but WRONG.
The needle uses actual injection pulse width and car speed to calculate the MPG. The system knows the AMOUNT of fuel being injected into the engine (pulse width x injector volume x fuel pressure) and knows the speed the vehicle is going at so the calculation is "elementary"..
At

Well, now you have a NEW experience. You might try finding actual data or facts, they help a lot in these sort of discussions. I have no idea why the system is programmed to "infinite" - but you might consider that to actually be "no MPG.." instead.
If you want to see instantaneous data - your on-board-computer can give you MPG (probably two different displays) - and pressing the reset button will effectively give you the instantaneous data. This is calculated from the same formula as drives the needle in your speedo.

How it works isn't effected at all by the miles on it. BMW has used the same meter and formula driving it since they introduced it back in.. probably early '80's.
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• posted on August 12, 2005, 4:20 pm
Infinite guages were required by Canadian law; US is the opposite - 0 mpg whilst standing still.
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• posted on August 12, 2005, 6:11 pm

You contend incorrectly. Here's why. If you were rolling to a stop, the needle will point to full scale Max. MPG, but when you are actually stopped, the needle points to full scale Min. MPG, but the manifold pressure hasn't changed.
You say it is pointed full scale MAX while not moving, but the fact is that it is pointed full scale MIN.
On my car, a '94 3 Series, the graduation on the scale is not nearly adequate to see the change you are talking about - the movement to show 10 mpg can be anywhere from 7 to 13 mpg.
The real way to check is to drive without the air on for a significant distance, then fill up and see what the mileage is, then drive again with the air off and fill up and see what the mileage is. My guess is that the two fill ups will be within 1 mpg of each other.
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• posted on August 12, 2005, 6:11 pm

But it goes to zero when the car comes to a stop but the engine still running? A vacuum gauge would read near maximum.
--
*Growing old is inevitable, growing up is optional *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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• posted on August 9, 2005, 11:52 pm

Pete just pulled your leg. Your last sentence has the "gallon" & "mile" reversed. I'm sure you meant to say the difference would be 1/4 mile per gallon difference, not 1/4 gallon per mile. :)
--

-bob-
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• posted on August 10, 2005, 6:22 am
Hello,
As our cars get older, scoring of the compressor surfaces will cause loss of cooling ability and create additional drag. This is because the esther oil has slowly escaped or is breaking down. Sure you could add some more esther but esther may not be enough... enter Molybdenum Disulphide.
If you were to go down to the local parts house today you will find products that promise 10 degree cooler air; a performance booster. Its simply more R134 and 2oz of moly. Moly acts like little bearings and reduce friction quite well. So give it a try. Add one of these "boosters" and see if things improve for you. It should but dont expect your guage to change.
As for your guage - not it is not vacuum sensing, its driven by the ecu. It is quite possible that the engineers programmed it for the worse case senario, thats when the belt driven fan is locked up. That could be another 5 -15 hp loss on top of what the compressor uses. Oh! And lets not forget the additional power the alternator needs to generate to run all of the fans too; that's another power loss!
;-) Blake