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Discussion Starter #1 (Edited by Moderator)
Have read quite a few threads on different forums about high def usage on 17-19 trucks. Mine does seem to use considerably more than my old 15. I have heard it may be a newer emissions requirement or such but I thought I would post what I have seen since I started watching it. The dealer says everything is good, no problems they could find but to record it.

Last 4436 miles. 305 gal diesel = 14.6mpg ( approx 50:50 towing and unloaded) used just over 10gal def which by my math is about 3.3% def per gal fuel used.
My last 650 mile trip was towing a light load (5k @70mph with a strong headwind almost the whole trip w/a higher profile). I averaged 11.5mpg , used approx 56 gal of fuel and just over 2.5 gal of def which is almost 4.6%.

My old 2015 would go way farther with a smaller def tank. Seems from memory, I was way closer to 1.5%? Anyone else try and keep track? I will be refilling again soon and keeping track till my next service (approx 3k miles) but that will be a hit as I don't think I have any trips soon.
 

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Not sure how to edit...Long weekend, should be DEF!!!
I went in and adjusted the post for you.
All you need is 8 more posts and your editing can begin

:thumb:
 

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Some 2020’s go through the entire tank of def fluid in just under 3k miles.
There has been three updates for DEF usage. Do you have the latest?

When ever I have a new vehicle when I go into service the first thing they do is see if there’s any updates per vin number.

What I have found is (2020 DURAMAX GMC & CHEVROLET) Facebook group has more info on the 2020’s than any diesel site that I belong to. And that’s a lot.
Join that group and in the search enter (DEF) lots to read
 

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The time I checked mine I went through 5 gal of DEF in 2400 miles of mostly towing. about 1 gal. DEF for 42 gal. of fuel.
 

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In 50K miles my LML has averaged 1 gallon of DEF for 1150 miles. Overal mileage for 50K miles is close to 13mpg. Or about 1 gallon of DEF for 89 gallons of diesel. Numbers I've seen indicate 1 gallon of DEF per 80-120 gallons of fuel.

1 gallon of DEF/40ish gallons of diesel seems high, maybe they are using less EGR to reduce NOx and hence need more DEF. I'd rather use more DEF and less EGR.
 

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In 50K miles my LML has averaged 1 gallon of DEF for 1150 miles. Overal mileage for 50K miles is close to 13mpg. Or about 1 gallon of DEF for 89 gallons of diesel. Numbers I've seen indicate 1 gallon of DEF per 80-120 gallons of fuel.

1 gallon of DEF/40ish gallons of diesel seems high, maybe they are using less EGR to reduce NOx and hence need more DEF. I'd rather use more DEF and less EGR.
I must be missing something here. My 2012 LML has used less than 3 gallons of DEF every 5,000 miles, which should be about 1 gallon of DEF per I650 miles traveled (about half towing). Am I to understand that the newer HD's are getting much less mileage of DEF than that? That doesn't make a lot of sense.
 

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I must be missing something here. My 2012 LML has used less than 3 gallons of DEF every 5,000 miles, which should be about 1 gallon of DEF per I650 miles traveled (about half towing). Am I to understand that the newer HD's are getting much less mileage of DEF than that? That doesn't make a lot of sense.
Why does that not make a lot of sense?

How much weight are you towing?
 

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My L5P gets the same mpg of def as my LML.


FWIW, following this and other 'dmax' forums since before we had def -- the 1650 mpg number would definitely be higher than what most report.
 

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DEF consumption in MPG of DEF useage is going to vary all over the map highly dependent on actual fuel MPG. IMHO a more consistent way to compare DEF useage is in gallons of DEF per 100 gallons of fuel. When I'm towing I go through about 1 gallon of DEF every 1K miles or less. But on the highway unloaded It's in the 1 gallon for 2K miles range. That's on a 2015.5 LML
 

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Why does that not make a lot of sense?

How much weight are you towing?
I towed a boat and trailer weighing around 18,000 lbs. And to be more accurate, it was between one-third to one-half of the mileage. I did replace the ECM after getting a DEF "poor quality" message at 28,000 miles.
 

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DEF consumption in MPG of DEF useage is going to vary all over the map highly dependent on actual fuel MPG. IMHO a more consistent way to compare DEF useage is in gallons of DEF per 100 gallons of fuel. When I'm towing I go through about 1 gallon of DEF every 1K miles or less. But on the highway unloaded It's in the 1 gallon for 2K miles range. That's on a 2015.5 LML
Yes, the def/fuel ratio should always be the same or close to the same.

GM suggested about 1.5 or so gallon def per 100 gallons of fuel in info sheets they published when the LML was introduced.
 

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Yes, the def/fuel ratio should always be the same or close to the same.

GM suggested about 1.5 or so gallon def per 100 gallons of fuel in info sheets they published when the LML was introduced.
Good info! Thanks to all for the feedback on this thread. I always learn something here.
 

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I didnt; watch the video (this time) but iirc, that vid is the approx 1000 mile drive to get the truck to Colorado from where they picked it up?

1 gallon of def test (using 2 different pumps) doesn't make a very accurate test, imho.

Too much room for 'fill error' -- even though they are trying.
 

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Excellent video test drive. The fuel economy is better than I expected, but the DEF mileage is lower for some reason.Thanks for the link.
DEF has come around to be the new Ethanol "Get rich" scheme..

The EPA raises its standards on emissions so that means more DEF usage and more $$ from the Consumer...
 
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There’s a TSB for DEF usage.
 
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