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We recently traded our 2014 LML for a 2019 L5P. The 2014 had over 326,000 miles all towing in a hotshot application. We averaged somewhere around 700 miles per gallon on DEF with both a 2013 and 2014 LML. So far the 2019 L5P is drinking DEF to the tune of less than 200 miles per gallon. With less than 5000 miles on the truck we have already dumped in 30 gallons plus whatever the dealer had in it. The fuel tank was full, don't know about the DEF. Typically the FIRST indication of DEF level less than OK is around 130 miles. Never get an alert at 1000 or 250. I presume this is because it is going through it so fast. This just doesn't seem right to me.
 

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These new trucks drink the stuff fast. I’m filling my tank every 3000k. Not real happy about it. I was hoping to never have the need to delete my truck but now plan to do so once some good options hit the market.


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This one is getting about 1100 miles per gallon of DEF, as calculated when refilled at about 6500 miles (it was still at about 22% iirc)

Since def used is directly related to ful used -- same truck is about 17.5 or 18 usmpg for the first 10,000 miles. (yes, I log every fill)

Currently at about 12,000 miles, still haven't added any more -- and still above 30%.



DEF used is a % of fuel so if it uses twice as much fuel, it should use twice as much def.
 

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This one is getting about 1100 miles per gallon of DEF, as calculated when refilled at about 6500 miles (it was still at about 22% iirc)

Since def used is directly related to ful used -- same truck is about 17.5 or 18 usmpg for the first 10,000 miles. (yes, I log every fill)

Currently at about 12,000 miles, still haven't added any more -- and still above 30%.



DEF used is a % of fuel so if it uses twice as much fuel, it should use twice as much def.


You are only partially correct yes def is a % of fuel but Manufactures may be required to lower the emissions further over the years so maybe requirements are different today than they were just a few years ago which means they may have to increase fluid usage. Or it maybe as simple as GM not wanting to be too close to the maximum allowed NOX level so is adding more Def fluid to be safe. Either way these trucks definitely use more fluid than in years past and it’s related to emissions requirements. Case in point is the Volkswagen TDI’s. All vehicles involved in the scandal immediately got an increase in def fluid usage as part of meeting the requirements. This was just one of the changes but was required in all TDI vehicles.


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I have seen other reports of very heavy DEF usage when towing with the L5P, maybe that is one reason for the increase in DEF capacity in 2017 to 7 gallons. The diesel supplement doesn't make a lot of sense either because it states when the 1,000 mile warning appears approximately 3.8 gallons are needed to fill so unless the warning is VERY conservative the 3.2 gallons in the tank at the 1,000 mile warning isn't expected to last very long. According to my math the manual indicates the 1,000 mile warning comes on with about 45% remaining in the tank (3.2 gallons left out of the original 7).

There is probably a way to make the warnings/level readout more asinine and less useful than current but it would take a room full of consultants to do so. With all of the electronic horsepower currently available a simple and fairly accurate readout of DEF tank fill level and the ability to use the trip computer to display average DEF consumption per 100 miles would make it a lot easier on owners to plan ahead.

I am pretty set on ordering a 2018 Denali to replace my 2006 Sierra SLT but stuff like this makes me sometimes wonder if I am about to make a mistake. The L5P equipped Denali would be my third Dmax powered Sierra and the first two have treated me very well.

We also don't know how low the tank is when the gauge reads 'zero'.

(is the full 7 gallons usable?)
 

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I've been keeping records of DEF usage since I got my truck. So far, 460 gallons of fuel required 10.4 g of DEF. That is a ratio of about 2.3% DEF to fuel. This data began once I filled the DEF in the truck after getting it home from the dealer since I did not know for certain what the DEF level was when I picked it up.

2.3% seems high compared to the LML values I just read here.
I will keep reporting on this as I drive, which is not but 10000 miles/y or so.

Hope this help.
 

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I have seen other reports of very heavy DEF usage when towing with the L5P, maybe that is one reason for the increase in DEF capacity in 2017 to 7 gallons. The diesel supplement doesn't make a lot of sense either because it states when the 1,000 mile warning appears approximately 3.8 gallons are needed to fill so unless the warning is VERY conservative the 3.2 gallons in the tank at the 1,000 mile warning isn't expected to last very long. According to my math the manual indicates the 1,000 mile warning comes on with about 45% remaining in the tank (3.2 gallons left out of the original 7).

There is probably a way to make the warnings/level readout more asinine and less useful than current but it would take a room full of consultants to do so. With all of the electronic horsepower currently available a simple and fairly accurate readout of DEF tank fill level and the ability to use the trip computer to display average DEF consumption per 100 miles would make it a lot easier on owners to plan ahead.

I am pretty set on ordering a 2018 Denali to replace my 2006 Sierra SLT but stuff like this makes me sometimes wonder if I am about to make a mistake. The L5P equipped Denali would be my third Dmax powered Sierra and the first two have treated me very well.
Well,

I can tell you on mine that I get the 30% message with 1000 miles left, but I always can put in 5 gallons no problem. I seem to have it down so I know how much to put in and the old stuff will be used. It never overflows with 5.
 

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I've been keeping records of DEF usage since I got my truck. So far, 460 gallons of fuel required 10.4 g of DEF. That is a ratio of about 2.3% DEF to fuel. This data began once I filled the DEF in the truck after getting it home from the dealer since I did not know for certain what the DEF level was when I picked it up.

2.3% seems high compared to the LML values I just read here.
I will keep reporting on this as I drive, which is not but 10000 miles/y or so.

Hope this help.
It’s because GM is trying to get more in the safety zone on emissions requirements. Two years ago several companies including GM got dinged on diesel emission levels. Going up on the DEF output just gives them more safety margin and keeping them more in the middle so to speak on NOX levels.
 

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I have a 2017 GMC towing a 16,000 lb 5th wheel RV it is 13' 6" tall. I towed it 65 mph for 2,100 miles and got 8.7 mpg. towing 3 days with 20+mph head winds. I filled the DEF tank when we left and got a message to add more before 300 miles. That happened after I had went 1,800 miles. So it used about 5 gallons of DEF to 200 gallons of fuel. On the return trip I got 10 mpg no major wind.
 

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Hi all. This DEF is very confusing to me. Here is my experience. Picked up truck with few hundred miles from dealership. Drove it till about 1300 miles and got DEF alerts, started with higher percentage, quickly dropped down to 29% than little after that to low DEF. I placed 2 jugs total of 5 gallons (2.5 each) and it didn't fill completely. Have been driving since, truck now show 4100 miles and DEF still shows OK message. So, I am wondering if usage has something to do with me not towing (I only tow my camper during camping season May-October) or the dealership didn't have DEF filled up as they should. I feel like the dealership had only 2.5 gallons in there therefore, I got alerts as it was approaching 1500 miles mark. Any feedback is appreciated.
 

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Hi all. This DEF is very confusing to me. Here is my experience. Picked up truck with few hundred miles from dealership. Drove it till about 1300 miles and got DEF alerts, started with higher percentage, quickly dropped down to 29% than little after that to low DEF. I placed 2 jugs total of 5 gallons (2.5 each) and it didn't fill completely. Have been driving since, truck now show 4100 miles and DEF still shows OK message. So, I am wondering if usage has something to do with me not towing (I only tow my camper during camping season May-October) or the dealership didn't have DEF filled up as they should. I feel like the dealership had only 2.5 gallons in there therefore, I got alerts as it was approaching 1500 miles mark. Any feedback is appreciated.
It has been a very common thing for many (most?) dealers to not fill the def tank at PDI.

Like the fuel tank, the def tank is not filled at factory.

This has caused endless confusion with def truck owners - that have concerns/stories like yours.

I would say/expect the 4100 miles since adding the 2 jugs, and still above 30% in tank is normal, and the earlier experience at 1300 miles was because your dealer didn't do their job.



DEF usage is in proportion to fuel usage -- more fuel used will mean more def used.
Usage about 1.5 to 2 gallons def per 100 gallons of fuel.
It's used whenever it's running.

The def level indicator in the dic will read 'OK' until the def level gets down to 30%.


Mine was about 6500 miles when first refilled. It's now over 12,000 and hasn't had a second refill.
I plan to wait til it gives at least the 1000 mile warning, and probably til the 300 mile warning. (just because)
 

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Same experience here, it is really heavy usage when towing. Truck was filled when I bought it. Had 1500 miles on it last week before I left on my trip. I took an extra 2.5 gallon jug I had in my shop with me. Sure enough, a few hundred miles in and it was out of DEF and got the 500 mile warning. Completed my 750 mile trip and it was down to 30 miles or so before going into speed reduced mode. Stayed the night and put in the DEF I brought in the morning and headed out. Couple hundred miles in and it was out of DEF again and back to the 500 mile countdown. Drove it a few hundred more miles and stopped and put in 5 gallons. Completed the trip with another 400 miles or so without it complaining but I am sure it is probably close to being empty. Insane how much this uses compared to my last 2 LMLs.
 

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That’s weird. Mine ran out within the first 2k miles of when I bought truck new this past October. Now even when towing several thousand pounds i can still go at least 6k between refills. Seems these trucks are all over the map. I’m currently at just over 7300 miles since last fill and the warning chime has yet to even come on. Lol


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That’s weird. Mine ran out within the first 2k miles of when I bought truck new this past October. Now even when towing several thousand pounds i can still go at least 6k between refills. Seems these trucks are all over the map. I’m currently at just over 7300 miles since last fill and the warning chime has yet to even come on. Lol


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You were likely another victim of 'dealer didn't fill def' at PDI.


'DEF level OK' could be 2 gallons or 7 gallons.
 

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You were likely another victim of 'dealer didn't fill def' at PDI.


'DEF level OK' could be 2 gallons or 7 gallons.

You sir are spot on. Dealer puts the minimum in to get you off the lot. I got 2200 miles on my first refill. Put 5 gallons in and I'm close to 8K on the ODO with no warning yet and DEF reads OK. I've taken two trips with the 15000 # Fifth wheel since refill and still have right at 6k miles on 5 gallons. 'dunno;
 

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I would normally agree with you guys on the dealer not filling. In my case, I ran out as I mentioned, put in 2.5 gallons, and was out again in about 400-500 miles. So, I ran out at about 1800 miles or so on the original fill, so by what I saw with the 2.5, that actually tells me it was possible it burned through it in 1800 miles. No way to be 100% sure of course, but I have never seen DEF usage like this.
 

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You sir are spot on. Dealer puts the minimum in to get you off the lot. I got 2200 miles on my first refill. Put 5 gallons in and I'm close to 8K on the ODO with no warning yet and DEF reads OK. I've taken two trips with the 15000 # Fifth wheel since refill and still have right at 6k miles on 5 gallons. 'dunno;
Not all dealers do that deliver them without adding (or with just adding some) more than the factory put in.

I believer mine was filled, and the dealer did provided an additional 2.5 gal jug.

Didn't add any til somewhere around 6500 miles, when I added the jug and topped it off at a bulk station so I could determine how much it was using (because so many were/are claiming L5P using more than the LML). Calc'd something like 1100 miles per gallon -- same as what my LML got.



At over 12,000 miles haven't added any more yet. (Expecting to get 1000 mile warning any time).



All that said, it doesn't mean that none of them are using more. Some probably are. But not all are using more.
 

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While I don't doubt there are some DEF guzzling trucks out there you have to have the correct sample size, e.g. don't judge it in a few thousand miles. Log it over say 10k miles and see what you get. Surely if your towing it is going to use more yet a good comparison again over longer time periods is how much fuel and then how much DEF. If you go 20mpg on normal highway and say 11.5 mpg when towing well that is a 70% increase in fuel consumption so common sense would tell us we need a 70% increase in DEF as well and perhaps it is more.

Long story short under non towing conditions we should be expecting 1000 miles/gallon of DEF w/o towing while towing would get you about 550miles/gallon of DEF. Try out those numbers over a proper sampling interval and see what you get.
 

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I have read through the many threads on DEF use on this forum for the current engine; this one seems to be related to mine. But I have not seen a post yet that indicates DEF use at the level we just saw on a trip west out of Houston. I am trying to determine if my recent high DEF use is common.
We have a 2018 2500 diesel towing an 8800 lb. travel trailer (11 feet tall, not a fifth wheel). We ran from Houston up through Wichita Falls, Gallup, Tucumcari and Flagstaff to the Grand Canyon. Then down through Phoenix, over to El Paso then to Fredericksburg TX and back to Houston. On the leg from Gallup to Fredericksburg we used 5 gallons of DEF. That is about 1400 miles of towing the rig and about 150 miles of unhooked driving near the Canyon and in Phoenix.
The truck computer showed about 11 mpg for that leg. It is essentially flat from Gallup to the Canyon, then down slope to Phoenix, and then relatively flat over through Texas. I did drive in cruise with TH at 68 mph. Weather was mild; not strong head or cross winds the entire trip. I did also check the truck mileage computer with a trip odometer and fuel receipts. Got the same average mpg. My cipher shows 140 gallons of diesel consumed.
I got the “add DEF soon or speed reduced” notice coming into Gallup (I did not get any of the early soft warnings). I added 5 gallons of DEF at the camp ground. Then continued on our way thinking DEF would be fine back to Houston. But outside Fredericksburg I again got the “add DEF soon or speed reduced” warning, again with none of the earlier soft warnings. Added 2.5 gallons DEF and took the truck into my dealer when we got back home.
Dealer checked for leaks, programming, etc. No results. They called GM at my request. “Rare but that high use might occur”. Also said that the early soft warnings might be skipped if the computer sees high DEF consumption and is concerned about low level coming up too quickly for the soft warnings.
So it appears we drove about 1550 total miles, used about 5 gallons of DEF in 40 gallons of diesel, and GM says “rare but maybe”.
Has anyone else seen these DEF usage numbers?
Houston might have a problem, so thanks for all input.
 

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a lot of stop and go will soot up the DPF, plus when its new I found it when into regen faster more often
 
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