What is your usage of DEF when towing. I have a 2020 GMC 3500 sierra 6.6 Duramax diesel.I have 17000 miles on it now. Most of the miles are regular driving hardly use any DEF.Had my truck serviced oil and filter change. Fuel filter changed as well as air filter changed. Dealer topped off with DEF. 3 days after service temperature got down around zero with a brisk wind. Received an message bad DEF 100 miles to slow down. Message disappeared on way to dealer when the truck warmed up. Dealer checked for codes and nothing. Next several weeks temperatures were in the teens. We left for Florida going through West Virginia and Virginia. Right before we got to Savannah received another message 1000 miles to reduced speed DEF. I had 2 1/2 gallons with me i filled it up. When we got to Florida I bought another 2 1/2 gallons and it took almost all of that. So I had just under 900 miles and that tank of DEF. On the way home I received the message again 998 miles i put 2 1/2 gallons in the gauge showed full. About 700 miles. Went another 375 miles and received message again 998 miles to slow down. Checked the gauge and still 3/4 full. I have an appointment for dealer to look at.Towing a 14,000 lb fifth wheel speeds of 60 to 70 MPH.

I don't have the problem with the codes but I do use about 2x the def when towing. Makes sense, your engine is working harder.

I think the general rule is about 1% and that is about what I get while towing. 100 gal of diesel = 1 gal of def.

doughogue1 wrote:I think the general rule is about 1% and that is about what I get while towing. 100 gal of diesel = 1 gal of def.With what you are saying I was probably a little more the 1 gallon per 100 gallons

doughogue1 wrote:I think the general rule is about 1% and that is about what I get while towing. 100 gal of diesel = 1 gal of def.yup DEF useage is basically determined by fuel useage. And the 1% number is a good "guestimate" On my 2015 Denali over 60K miles average 1 gallon of DEF for 90 gallons of fuel.

I the last sentence you stated you have a gauge, just check it for level. The 1,000 miles to slow down just should not show up.I would expect a 100 mile warning anything else is a bug.Last thing DEF will freeze, so might be the reason for warnings, the heater may not have thawed it fast enough.

DEF when towing | Good Sam Community

Toolguy5
Explorer III
Mar 08, 2022
just picked up truck from Dealer. Could not find anything wrong with my DEF system. GM did issue an article revised February 3rd 2022 on Excessive Diesel Exhaust Fluid. The gauge is only a reference and may need several key cycles to show actual amount in tank. The rate of DEF will vary on Temperature, Altitude, speed and load. So the harder engine works the more DEF.
They Also stated as the NOx reduction requirements increase the more the newer vehicles will use. I guess 2020 was a reduction requirement year.
Article also states that if you receive message of XXX amount of miles that may not be accurate as well. It could decrease or increase depending on conditions and load of engine.
ksss
Explorer
Mar 07, 2022
I would agree that the '20 uses more DEF than the older trucks use. Same L5P engine, but they clearly dialed up the DEF.
mkirsch
Nomad II
Mar 07, 2022
DEF freezes when it's extremely cold. So I would expect some unexpected results when the DEF is frozen which clear up after the DEF thaws out.
Toolguy5
Explorer III
Mar 07, 2022
Huntindog wrote:
I am now on my third Duramax.
My current one is a 2020. I do not use them for daily driving. They have all been 90%+ towing. I can say that this one uses a lot more DEF than the others.
GM obviously expected that, as this one has a larger tank.
It is what it is.

Having fun costs money. How much fun do you want to have?

That is what I am noticing but i will find out if i have an issue
Grit_dog
Navigator
Mar 07, 2022
We’re now almost 1/4 of the way through the 21st century.
Self driving vehicles actually exist, and the big 3 can’t figure out how to incorporate an accurate gauge for a consumable fluid, that is slightly more important than, say, windshield washer fluid!
Maybe glass sight gauges shouldn’t be a thing of the past! Lol

On a different note, I wish the def gauge in our truck was only part of the information screen and not an actual gauge. Because it looks like I have a broken gauge in the cluster since it’s been sitting below E since 2016….
Huntindog
Explorer
Mar 07, 2022
I am now on my third Duramax.
My current one is a 2020. I do not use them for daily driving. They have all been 90%+ towing. I can say that this one uses a lot more DEF than the others.
GM obviously expected that, as this one has a larger tank.
It is what it is.

Having fun costs money. How much fun do you want to have?
Cummins12V98
Explorer III
Mar 06, 2022
FishOnOne wrote:
With recent fuel prices skyrocketing, not sure if DEF consumption is a really big deal.

At least they include lube.
FishOnOne
Nomad
Mar 06, 2022
With recent fuel prices skyrocketing, not sure if DEF consumption is a really big deal.
Toolguy5
Explorer III
Mar 06, 2022
I also carry jugs of DEF when towing so I have it.

But when your gauge is showing almost a full tank and you get a warning low DEF something else is going on.

Hardly use any DEF when not towing. Usually top it off at oil changes.
Cummins12V98
Explorer III
Mar 06, 2022
Lantley said: "If lack of DEF gauge is the biggest negative of my truck than I'm doing OK!"

I agree! Really can't be that big of a deal to simply add to it based on the type of driving you have been doing. Always have some fresh water to rinse away any spilled DEF. If using a funnel or reusing the fill tube provided be sure to rinse thoroughly.

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DEF when towing

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