DEF Gas Milage Impact - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 14 (permalink) Old 08-02-2011, 02:46 PM Thread Starter
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DEF Gas Milage Impact

To test the impact of fuel economy I ran my DEF down until I started receiving the XXX miles until vehicle limited to 55 mph warning. During this time my fuel economy dropped down about 1 mpg to a steady 14.5 mpg. I ran this way for almost 50 miles and filled back up. Have not seen any change in fuel economy so my guess I need to complete a full regen then my mpg's should line back up.

So I would surmise from my short test that the DEF does currently play a role in fuel economy.

This got me thinking with the cost of DEF at about 7.50 a gallon are there any home brews available of the 32.5% urea solution to 67.5% distilled water solution? Seems like it would be more beneficial to have a tablet you could dissolve in distilled water and this should cut costs and increase shelf life and not be as susceptible to the temperature extremes recommended in retaining shelf life of the DEF solution.

Any thoughts?
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post #2 of 14 (permalink) Old 08-03-2011, 07:23 PM
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Def cost /gallon

Yea man, unless my math skills are not up to par, I figure @ 7.50/gal and a range of 1500 miles/gal my def expense equates to .005/mile.
(Just in case....That is a 1/2 penny/gallon)
You may want to raise the bar a little in respect to your cost reduction expectations.


Last edited by Starwarrior; 08-03-2011 at 07:31 PM.
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post #3 of 14 (permalink) Old 08-04-2011, 03:23 PM Thread Starter
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Originally Posted by Starwarrior View Post
Yea man, unless my math skills are not up to par, I figure @ 7.50/gal and a range of 1500 miles/gal my def expense equates to .005/mile.
(Just in case....That is a 1/2 penny/gallon)
You may want to raise the bar a little in respect to your cost reduction expectations.

I guess that is one way of looking at it... just remeber your eqation when you spend the coin to fill up the def tank...
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post #4 of 14 (permalink) Old 08-05-2011, 02:43 PM
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before i deleted mine and went straight piped i was buying the urea at a local gas station for around $4.70/gal. cost of this stuff is so minimal, especially when you figure how much better fuel mileage we get stock than 6.4 did (pre urea)

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post #5 of 14 (permalink) Old 08-13-2011, 05:09 AM
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ey guys...Might be a dumb question but do I need to keep an eye on the level of the fluid myself? If so how...or will the truck let me know when the fluid is low and I need to add? I have about 2200 miles on the truck now.

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post #6 of 14 (permalink) Old 08-13-2011, 09:40 PM
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the DEF is injected down stream of everything and it is supposed to make regens happen less frequently, so how does it cause a loss of fuel mileage. also as to monitoring the level. GM computer monitors and tells you when you are 1000 miles left then at 500 miles it warns at every 50 or 100 miles. so you have plenty of warning.

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post #7 of 14 (permalink) Old 08-15-2011, 04:56 PM
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Well my truck has over 80k on it now, in only 7 months of driving. I work it like a dog.

I notice that after a regen my mpg is noticeably better, especially empty. Although as of late i am noticing regen is more frequent, around 250 miles. This is due to the exhaust restriction pre regen.
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post #8 of 14 (permalink) Old 08-15-2011, 06:37 PM Thread Starter
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Originally Posted by buffalobob View Post
the DEF is injected down stream of everything and it is supposed to make regens happen less frequently, so how does it cause a loss of fuel mileage. also as to monitoring the level. GM computer monitors and tells you when you are 1000 miles left then at 500 miles it warns at every 50 or 100 miles. so you have plenty of warning.
The Ford system also warns you and provides plenty of feedback when your DEF tank gets low and finally runs out.

The point of my original post was to confirm what you have also stated that the DEF is supposed to cause less regens which should yield better fuel economy as unburnt fuel is dumped into the exhuast stroke during regen. I was able to confirm this as my mpg did drop down 1 mpg while running on an empty DEF tank; therefore I would attribute this to more regens while running without DEF.
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post #9 of 14 (permalink) Old 08-15-2011, 06:47 PM Thread Starter
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Originally Posted by BocaCobra03 View Post
ey guys...Might be a dumb question but do I need to keep an eye on the level of the fluid myself? If so how...or will the truck let me know when the fluid is low and I need to add? I have about 2200 miles on the truck now.
The system will keep you informed when DEF needs to be added, during that time you can run it down or in my case empty and you will continue to get notice that you have XXX miles until you are limited to 55mph. I didn't push mine to the point I was limited to 55mph as the next limit the system evokes is an idle only mode.

It may seem like common sense but keep in mind that there are two fillers necks behind the fuel door cover and you don't want to put DEF in the diesel tank, use the blue one for DEF or it will cost you...
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post #10 of 14 (permalink) Old 08-17-2011, 04:01 PM
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Re: Cost of DPF

When it came time to add DPF, I went to my local dealer and bought 3 gallons for about $8.50/gallon. That was wholesale! So I Googled "DPF" and found that it can be purchased for a lot less. Now I buy it at my nearby O'Rielly's. $12 for 2.5 gallons or about $5/gallon. I check the "Info" panel and refill when it is below 1/2 tank. Works for me!
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