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

Thermoguy
Explorer II
Mar 06, 2022
I fill my DEF tank before I take it in for an oil change every 5000 miles. Mostly towing and local driving. 2015 Chev 2500. I buy 2 of the 2.5Gal jugs but can not fit both of them in. I have only seen the low DEF waring once and purchased the truck new. Didn't stop or anything, just waited until I got home and filled as I had an oil change appt the next week.
Michelle_S
Explorer III
Mar 05, 2022
I average 450 to 500 miles per gallon of DEF pulling 19.5K.
I've never gotten the 1000 mile warning, generally get the warning that I have 250 to 400 miles before DEF Empty.
rhagfo
Explorer III
Mar 05, 2022
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.
ktmrfs
Explorer II
Mar 05, 2022
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.
Toolguy5
Explorer III
Mar 05, 2022
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
00Buckshot
Explorer
Mar 05, 2022
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.
way2roll
Navigator II
Mar 05, 2022
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.

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DEF when towing

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