MPG increase with temperature: 1.4% for every 10ºF

Bob_Fout

Oil Wanker
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Sep 5, 2004
Location
Indiana
TDI
2003 Jetta - Alaska Green (sold) / 2015 GTI 2.0T
Hmm... I seem to have dropped mpgs (3-4mpg down) since it got into the high 90's to 100's here in the south. Yet I drive about the same. Can something be wrong? It just seems too much.
Compared to what previous temps? A/C usage more?
 

Enabled

Veteran Member
Joined
Apr 23, 2013
Location
Houston, TX
TDI
2003 Jetta TDI Manual, BMW 328d SW
Compared to what previous temps? A/C usage more?
I had best mileage in the late winter/early spring (~70 degrees F). In fact I was even able to hit 50mpg even during a drive from Aspen Colorado to Houston, TX during the Polar Vortex blizzard in late December (0-10 degrees F), on winter tires (only Houstonian with winter wheels available??), but at max 50mph due to conditions.

But it has been steadily declining since then, as it was warming up. I was able to keep 47mpg tank average for a while in the spring, but then it declined. My last tank was 43mpg even though I was trying to shift at 2000 rpm when possible, in a small attempt to keep it up. I still drive with a heavy foot reaching 85mph often, but I haven't changed that. Obviously I've been using AC more, but 3-4 mpg? Maybe that's normal in addition to my heavy foot driving.

I have .681 gears as well.

The car makes good power (as good as it's gonna get with PP520's and VCDS tweaking) low smoke on WOT, and monitoring in VCDS for engine health is very good (per cylinder IQ deviation is good, everything else is normal)

I'll try MAF cleaning and connector servicing, other than that, all filters are new, and all fluids in powerline/driveline are new.
 

Diesl

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Dec 3, 2012
Location
Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Slowly decreasing tire pressure? I know mine has been slowly creeping down from 45 psi in the spring to 42 psi two weeks ago. Now the dealer 'corrected' the pressure at the 30k service, and I reset it to 42 psi.
 

ksing44

Veteran Member
Joined
Feb 13, 2010
Location
Southeast PA
TDI
2010 Golf TDI
My winter plunge in mpg every January- February is quite obvious with a lifetime fuelly graph. I climbed out of the hole very nicely so far this year, but unfortunately I'll be descending any time now as we go into autumn in Pennsylvania. I have to admit there is also a contribution from holiday vacation time and a little less commuting around the end of the year, but my trends in mpg are clearly affected by temperature. Last winter was cold for longer than normal and my mpg was lower for longer too!


from Flickr


PS: I run my big fat 235/45/17 Grand Touring All-season tires at 33 psi and still averaged just over 40 mpg for almost 100K miles. And that's not even taking into account that my mpg is actually just a bit better because my speedometer/odometer is off by about 1.4% with my bigger tires. I don't understand how some of you guys can stand driving with such hard tires for maybe a teeny weeny bit of difference in mpg. It must be so noisy and uncomfortable too.
 
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SkeeterMark

Veteran Member
Joined
Jan 8, 2013
Location
North Branch, MN
TDI
2013 Jetta TDI 6M
Here's another chart to demonstrate the effects of temperature (red line). Note the last entry... we had a cool week during my last tank and you can already see the drop in mpg. Note there are no additives for winter in the fuel yet. I believe the effect of winterizing additives is also minimal, in comparison to just the temperature alone.

Also, MN went to B10 in July, and there does not appear to be a noticeable difference - in fact I had my record mpg on it.

Also in this graph, the green line is what my MFD reports, compared to actual calculated mpg (blue).




*Temperatures are calculated by taking the start and end temp of each drive, and weighted for the mileage of that trip to get and average temperature while the car was in use. I used to just take a daily average temperature off accuweather, but I'm too anal to do that, though the variance in the two methods (don't ask how I know) is usually negligible because my drive to and from work actually coincides with the low and high temperatures for most days.
 
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Diesl

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Dec 3, 2012
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Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Super, more plots!

Mark, if you stretch the MPG/MFD vertical axis a bit (maybe by chopping off zero, and running it from 30 to 60 mpg) then one could see the variations in mpg better.
 

puntmeister

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Feb 3, 2013
Location
Arizona
TDI
2004 Jetta BEW
Haven't had time to read through the whole thread yet, so this may have already been addressed, but my 2 cents:

"Temperature" needs to be specified as to the temperature of what. And it needs to include air pressure & air density as well.

As the engine is warmer, it operates more efficiently. Clear enough - warmer engine temp = better mpg.

As air temperature goes up, air density typically goes down. Less dense air would genearally result in less efficient engine operations.

Of course, as air density goes up, so does aerodynamic drag.

It all gets too complicated for me, as I am not an engineer by any stretch - I am just applying what I have learned & experienced from flying airplanes (where temperature, air pressure, and air density all have noticable impacts on a plane's operations).
 

puntmeister

Veteran Member
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Feb 3, 2013
Location
Arizona
TDI
2004 Jetta BEW
As for the lower MPG in winter in Pennsylvania - could be several issues here, only indirectly related to temperature:

1) Winter diesel formulas, which, everyone seems to be certain of, are less energy dense than summer formulas.

2) I've been to Pennsylvania during many winters: lots of snow. Snow on the roads results in more sporatic driving - more stop & starting, driving in first & second gear, etc - which may lead to lower MPG.

In response to another post: AC uses a hoopload of power. I've done VCDS logs to determine fuel consumption of AC at various speeds - .6 liters/hour, at whatever speed. I also ran the numbers as MPG at various speeds - with/without AC - my logs showed a much bigger hit than 3 or 4 mpg - if I remember correctly, more like 6 mpg - but that varies. Consumption per hour doesn't. Of course, I'm in Arizona - so, when the AC is running, its running full-on.
 
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Diesl

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Joined
Dec 3, 2012
Location
Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Welcome to the thread! I include a few answers below.
Haven't had time to read through the whole thread yet, so this may have already been addressed, but my 2 cents:
"Temperature" needs to be specified as to the temperature of what. And it needs to include air pressure & air density as well.
Absolutely! I'm talking about air temperature. Somewhere in the beginning I specify where I get my temperatures from. Air pressure doesn't vary so much seasonally where I live. I think that's true for most of Earth. Density is inverse proportional to (absolute) temperature; somewhere in the early posts I realized it varied by 7 or 8 percent between winter and summer in my area (upper Midwest). Huge factor for highway driving.
As the engine is warmer, it operates more efficiently. Clear enough - warmer engine temp = better mpg.
For me warmup is a small fraction of my driving. Of course this can be very different for others.
As air temperature goes up, air density typically goes down. Less dense air would genearally result in less efficient engine operations.
My car has a turbo and a little computer that tells it what inlet manifold pressure it would like to see. (Most here do on the TDI forum... :) ) As long as the turbo doesn't reach its rpm limit, it will compensate for atmospheric air pressure changes. That's why people at higher altitude really like these turbo diesels.
Of course, as air density goes up, so does aerodynamic drag.
Absolutely! For me this factor alone (air drag change multiplied by my highway fraction) is sufficient to explain most of my summer winter mpg difference.
It all gets too complicated for me, as I am not an engineer by any stretch - I am just applying what I have learned & experienced from flying airplanes (where temperature, air pressure, and air density all have noticable impacts on a plane's operations).
The trickiest part is good record keeping. Anybody can learn how to do that.
 

SkeeterMark

Veteran Member
Joined
Jan 8, 2013
Location
North Branch, MN
TDI
2013 Jetta TDI 6M
Super, more plots!

Mark, if you stretch the MPG/MFD vertical axis a bit (maybe by chopping off zero, and running it from 30 to 60 mpg) then one could see the variations in mpg better.
Ya, that chart has the temp charted in there too, so it makes the vertical scale pretty tall.

But here's a chart of just the mileage (blue line). The red line plot is what you would have to achieve for mileage in a gasoline fueled vehicle, based on the prices I paid for diesel at each fill and the price for RUG at the same station at the time of that fill. It still shows the ups and downs of the seasons, which anyone in a cold weather climate will see.

 

Diesl

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Dec 3, 2012
Location
Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Air drag should be identical for identical car body shape at the same speed.
 

Diesl

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Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Updated post #5 with fall/winter 2014/15 chart. 40% done with gaspod experiment; so far no significant gain or loss in diesel mileage visible.


Histogram of diesel MPG with pods on and pods off (rear placement):


I moved the pods forward after three tanks full in this position, to be more in line with recommended placement.


For comparison, here are the histograms for two Golf TDIs with significant improvement due to gaspods:
data from http://www.gaspods.com/wp-content/uploads/Results-2012-VW-Golf-TDI.pdf


GaryTDI's Golf:
data from http://www.fuelly.com/car/volkswagen/golf/2012/GaryTDI/144378

 
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Diesl

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Dec 3, 2012
Location
Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Gaspod MPG vs Temperature

Same plot, for the gaspods placed 8" forward from the end of the roof:


Histogram of diesel MPG with pods on and pods off for the 8" forward placement:


So far this does not look much better than the rear placement; a significantly positive (or negative) result seems unlikely. Maybe the pods don't do much for the longer wagon, with the standard equipment roof rails. They clearly seem to work for the standard Golf TDI.
ADDED April 5, 2015: After six tanks without pods and five with, there is no significant difference in diesel mileage. The top graph shows that both sets (with and without pods) cover the same air temperature range, with similar temperature distributions.

August 2, 2015: Final verdict: the pods are not working for my Jetta Sportwagon. I'm suspecting that the roof rails create enough of a disturbance to fill in the pressure hole behind the hatch. I'm going to track mileage with the rails as-is and filled in (with styrofoam inserts).
 
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Diesl

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Dec 3, 2012
Location
Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Just added the last two months to the plots in the previous post. At some point I might create a separate histogram just for the summer peak, to see whether anything pops up.
 

Diesl

Veteran Member
Joined
Dec 3, 2012
Location
Chicago
TDI
'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Roof rail filler effect

I'm done with the gaspods. Now I am tracking the influence of filling in the roof rails (with styrofoam inserts).




Early conclusion: filling in the rails does not seem to significantly improve diesel milage.
 
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Diesl

Veteran Member
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Dec 3, 2012
Location
Chicago
TDI
'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
pods + rail fillers

I started to check the effect of pods + rail filler, versus rail filler alone.

The hypothesis gets pretty convoluted though: to have any hope for a positive effect one would have to assume that

  1. the rails on the JSW create too much turbulence,
  2. that with filled rails there is too little turbulence/mixing of the boundary layer (leading to a low pressure pocket behind the car, i.e. same situation as in a normal non-wagon Golf),
  3. and that the pods with filled rails create just enough turbulence to fill in the low pressure pocket behind the car, but not enough to make the drag significantly worse.

After looking at the first three data points I decided that this is unlikely to lead anywhere, and that I'm done testing.



Does anybody want the pods for testing purposes?
 

Diesl

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Dec 3, 2012
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Chicago
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'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Updated post #5. The best correlation seems to be between mpg and speed, with higher mpg for higher average speed. I assume this means it's simply an annual variation in city vs highway driving. Superimposed on these annual variations is a steady decline of diesel mileage for my car, from about 37 mpg in 2013-2014 to 34 mpg in 2019.
 
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jerrymander

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Feb 11, 2012
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Cowford, FL
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2011 Golf Reborn Edition, 2015 Golf, 2011 Golf w/ Factory Crushed Option
Diesl, I used to collect all my data in spreadsheets when I first got my car. Then I realized that it didn't really matter too much to me personally. I did collect a few bullet points with my old manual CR140:

45 MPH @ 6th gear: 42 MPH w/o drafting @ -30 F (Dakota winter, heat ON) (car would idle at 1400 it was crazy to me)
45 MPH @ 6th gear: 57 MPG w/o drafting @ 70 F (Florida winter)
45 MPH @ 6th gear: 59 MPG w/o drafting @ 95 F (summer)

75 MPH @ 6th gear: 33 MPG w/o drafting @ -30 F
75 MPH @ 6th gear: 42 MPG w/o drafting @ 70 F
75 MPH @ 6th gear: 46 MPG w/o drafting @ 95 F
75 MPH @ 6th gear: 50 MPG w/o drafting @ 115 F (Nevada summer)

75 MPH @ 6th gear: 45 MPG w/ drafting @ 70 F
75 MPH @ 6th gear: 50 MPG w/ drafting @ 95 F

My 2015 and new(er) 2011 do similar. I hit peak fuel efficiency when the temperatures hit triple digits. In the beginning, I vigilantly hand calculated tank averages and found the multifunction display calculations to be reasonably accurate (<2% error). I stopped collecting this data meticulously around 2014. I almost never used my aircon. In Dakota I sporadically filled up with B10/B15 but never actually kept track of what was what.

Your decline is neat: I never noticed a trending MPG drop on my 2011 from 2011 to 2017 @ 112,000 miles, but I had no factory emissions equipment.
 

ksing44

Veteran Member
Joined
Feb 13, 2010
Location
Southeast PA
TDI
2010 Golf TDI
Superimposed on these annual variations is a steady decline of diesel mileage for my car, from about 37 mpg in 2013-2014 to 34 mpg in 2019.
^^I also experienced a steady decline in mpg, even though the variations caused by changes in my commute, how fast I travel, and temperature have a bigger effect.


on Flickr
 

bobt2382

Veteran Member - TDIClub Contributor
Joined
Aug 1, 2009
Location
NJ
TDI
2010 CW GOLF 4DR 6MT TDI
Diesl, I used to collect all my data in spreadsheets when I first got my car. Then I realized that it didn't really matter too much to me personally. I did collect a few bullet points with my old manual CR140:
45 MPH @ 6th gear: 42 MPH w/o drafting @ -30 F (Dakota winter, heat ON) (car would idle at 1400 it was crazy to me)
45 MPH @ 6th gear: 57 MPG w/o drafting @ 70 F (Florida winter)
45 MPH @ 6th gear: 59 MPG w/o drafting @ 95 F (summer)
75 MPH @ 6th gear: 33 MPG w/o drafting @ -30 F
75 MPH @ 6th gear: 42 MPG w/o drafting @ 70 F
75 MPH @ 6th gear: 46 MPG w/o drafting @ 95 F
75 MPH @ 6th gear: 50 MPG w/o drafting @ 115 F (Nevada summer)
75 MPH @ 6th gear: 45 MPG w/ drafting @ 70 F
75 MPH @ 6th gear: 50 MPG w/ drafting @ 95 F
My 2015 and new(er) 2011 do similar. I hit peak fuel efficiency when the temperatures hit triple digits. In the beginning, I vigilantly hand calculated tank averages and found the multifunction display calculations to be reasonably accurate (<2% error). I stopped collecting this data meticulously around 2014. I almost never used my aircon. In Dakota I sporadically filled up with B10/B15 but never actually kept track of what was what.
Your decline is neat: I never noticed a trending MPG drop on my 2011 from 2011 to 2017 @ 112,000 miles, but I had no factory emissions equipment.
My experience concurs with yours, the hotter the weather the better the MPG. Also, the wind strength and direction would significantly impact MPG. Very noticeable during trips from San Diego to 29 Palms CA and to Reno NV on Rt. 395.
Thanks for posting your data.
 
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