- Sep 5, 2004
- 2003 Jetta - Alaska Green (sold) / 2015 GTI 2.0T
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.Compared to what previous temps? A/C usage more?
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.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.
For me warmup is a small fraction of my driving. Of course this can be very different for others.As the engine is warmer, it operates more efficiently. Clear enough - warmer engine temp = better mpg.
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.As air temperature goes up, air density typically goes down. Less dense air would genearally result in less efficient engine operations.
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.Of course, as air density goes up, so does aerodynamic drag.
The trickiest part is good record keeping. Anybody can learn how to do that.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).
Ya, that chart has the temp charted in there too, so it makes the vertical scale pretty tall.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.
^^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.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.
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.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.