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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old June 2nd, 2015, 12:19   #1
JosephJP
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Default Mpg with a/c

With the A/C off my average mpg on my way to work is around 50. With A/C on it's around 40. Does this sound normal?
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Old June 2nd, 2015, 19:05   #2
goodysgotacuda
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How much of an average are we talking about? over one tank ac on and ac off? over 20 tanks each?

Is the averaged based on the display or hand calculated?
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Old June 4th, 2015, 19:22   #3
JosephJP
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Average is the display read out for the trip. I haven't been able to do an actual calculation yet. I've only filled it up once so far.
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Old June 5th, 2015, 09:03   #4
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If you're referring to your brand new 2015, I wouldn't pay too much attention to the mpg yet.

Just record it in a log book or spread sheet. Take the average when you get to ~10~20 tanks

I would recommend doing hand calculations. Miles driven/gallons to fill up.
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Old June 7th, 2015, 19:11   #5
goodysgotacuda
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Quote:
Originally Posted by JosephJP View Post
Average is the display read out for the trip. I haven't been able to do an actual calculation yet. I've only filled it up once so far.

The numbers on the display are notoriously inaccurate, and checking within trips isn't a good way to make much of a definitive answer. It's like measuring a football field with a 6" ruler. You're going to be way off when you get to the other end..

Even averaging 20 tanks ambient conditions, traffic, wind, driving habits, etc will likely not net a good result for just testing the difference between the a/c on and off. Way too many variables and not many controls. The only reasonable way for you to get a decent delta would be to run a leg of a road multiple times with the a/c on, 1/2 tank worth and fill it up. Then do the same with the a/c off. Conditions will be as good as you can be and the vehicle will not have changed like it will in 12,000 miles [~20tanks].
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Last edited by goodysgotacuda; June 7th, 2015 at 19:15.
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Old June 7th, 2015, 20:36   #6
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Use the A/C if you're hot and uncomfortable.
Regulate temperature with the blower speed rather than the temperature dial. (warmer than full cold just uses the heater core to warm the air back up once it's been cooled down, whereas less airflow will make the TXV flow less refrigerant to maintain the same evaporator temperature, and the adjustable displacement compressor will therefore pull less load on the motor)
Worry less about the cost.
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Old June 7th, 2015, 21:54   #7
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For our 2002 & 2004 models, the full-cold is the way to go - 486 is right, any warmer than full cold just diverts some air over the heater core.

On a 2015 model - not sure this is the case. Maybe. But I figure they woulda advanced the tech a bit - fully variable speed, electric compressor (not driven by serpentine belt).

As for MPG - on my 2004 BEW, I ran some fairly clear-cut tests using VCDS, and straight stretches of road, at various speeds, with & without A/C - found A/C uses about 0.2 liters/hour diesel, regardless of speed. At highway speeds, works out to a 10% hit. At lower speeds, the % will be higher.
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Old June 9th, 2015, 04:53   #8
Rico567
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We have 28XXX miles on our 2013. Fuel average can be seen in Fuelly below, recorded since new. I have not done any record keeping on having the A/C on/off, but estimates based solely on observing the MFD readout on trips* would indicate a marginal difference, perhaps @1-2 mpg. This is insignificant to us, so I'm probably just going to start leaving the system in "Auto" with the A/C button lit, and let the climate control take care of it.

*We take a regular trip every two weeks of almost exactly 200 miles total, starting early in the morning, returning in the afternoon, so during the summer I have been turning the A/C off on the morning leg and on during the return in the afternoon. Since the MFD has reset in between, this makes it easy to observe.
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Old June 9th, 2015, 09:49   #9
gloaming
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Quote:
Originally Posted by goodysgotacuda View Post
The numbers on the display are notoriously inaccurate, and checking within trips isn't a good way to make much of a definitive answer. It's like measuring a football field with a 6" ruler. You're going to be way off when you get to the other end..

Even averaging 20 tanks ambient conditions, traffic, wind, driving habits, etc will likely not net a good result for just testing the difference between the a/c on and off. Way too many variables and not many controls. The only reasonable way for you to get a decent delta would be to run a leg of a road multiple times with the a/c on, 1/2 tank worth and fill it up. Then do the same with the a/c off. Conditions will be as good as you can be and the vehicle will not have changed like it will in 12,000 miles [~20tanks].
I wouldn't agree that it's notoriously inaccurate. The MFD has actually always been spot on for me. I hand calculated each tank for a year and was always within 1-2 mpg from the MFD. In fact my mfd was a bit conservative.

Everything else you said is true though.
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Old June 16th, 2015, 07:52   #10
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I notice that my MFD will show a dramatic drop in MPG when I kick the AC on. I normally just cycle it on and off manually for my commute and select times when I'm coming to a stop, slowing down or on down hills. Its not the best solution, but it works. I get a little hot some times, but when you put almost 30K on the ODO a year, every little bit helps.

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Old June 18th, 2015, 08:01   #11
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I loose 3-4 (10% ish) MPG running the A/C a lot. It sure beats sweating, though.
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Old June 20th, 2015, 06:46   #12
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How does the AC work on a MK4? When you select AC does the compressor stay engaged the entire time? Or does it cycle on and off?
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Old June 20th, 2015, 09:33   #13
[486]
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Quote:
Originally Posted by Wankel7 View Post
How does the AC work on a MK4? When you select AC does the compressor stay engaged the entire time? Or does it cycle on and off?
Stays on unless the low pressure cutoff on the low side comes into play. Internal junk regulates the displacement of the compressor to maintain a constant pressure differential.

If yours is cycling, you've got an issue.
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