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
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.
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
I don't know what numbers you were running, but you were not running a
quarter gallon per mile.
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...
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.
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
Going from 20 to 17 mpg, means he'd be using 2.5 liters more per each 100
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.
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.
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"..
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.
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
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.
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
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!
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