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Old September 2nd, 2013, 16:40  
Veteran Member
Join Date: Dec 2012
Location: Chicago
Default MPG as function of air temperature

The pattern doesn't fit winter diesel. It's a steady increase, not a sudden step:

It could be though some sensor drifting. [I guess I'll find out when it gets cooler again, which should be soon:]
Update 2016-12-31: avg. speed added

I had never looked at how much the drag loss actually changes due to air density change with temperature. Guess what: going from 30F to 70F lowers air density by 8%! Density enters into the drag power linearly. I do about 70% of my driving on the highway, where air drag dominates. 0.7*8%~5.5%! That almost fits too well....
Update Sep 5: Added a graph that shows five tank average mpg versus five tank average temperature, and hopefully updates automatically from the spreadsheet. (The graph in the first post won't update, and uses the average daily temperature for the middle tank.)

Update Nov. 8: it is getting cooler, and the diesel milage is dropping. Temperature vs time chart added.
Dec 16: use average for days actually driven.
Dec 21: Correcting some of the earlier temperature averages; e.g. Nov 30 - Dec 11, 2012 went from 48F to 41F.
Update 2014-2-28:
Disentangling the last plot by season:
First the falling temperatures of winter 2012/2013:

Maybe the initial rise (from right to left) is the running in of the engine??

Then the rising temperatures of spring and summer 2013:

The falling temperatures of fall and winter 2013/2014:

The drop in MPG follows a higher MPG curve than the preceding rise during spring and summer.
Spring/Summer 2014:

Fall/Winter 2014/2015:

Summer 2015:

Update 2016-12-31: I added average speed. Unfortunately there are two competing factors at work: the ever increasing highway speeds in the Chicago area, and a change in this car's mix of highway vs. city driving. It looks like city driving is winning, and dragging my average down.
If it works, take it apart and find out why!

Last edited by Diesl; December 31st, 2016 at 07:46. Reason: add speed
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