DEF Usage
 

My 2016 6.7 has about 25K miles. Always get the DEF 1/2 full message at about 3-3,500 miles, most times it won't take the entire 2 & 1/2 gallons unless I run it another 500 after the message. Got the less than 1/2 full message just prior to leaving on a trip & filled it. Towed a 14,000lb 5th wheel a total of about 1,300 miles the first 1,000 miles outside temp between zero & 10 above. Got the less than 1/2 full message at 1,600 miles. Does it use more towing in the cold? I always fill it with the Ford fill tube.
John

The trucks use DEF when you have high internal combustion temps. Basically high EGTs. DEF is used to reduce the NOx gases produced at high combustion temps.

Consider when you get high EGTs. It is when you have turbo boost. If you idle around town with no load at 1200 rpm, your EGTs will stay at 450°. Even driving empty on the interstate at 70 mph = 550° EGTs.

But when you hook up a trailer and tow it down the interstate at 70 mph, My EGTs jump up to 750-900° Climb a big hill and they are 1150-1200°

So you will consume way more DEF when you tow.

My experience is normal driving is about 1000 or mi/gal of DEF. When towing heavy, at highway speeds, this drops. From the Diesel supplement:


Aggressive driving/trailer towing: 2,800-5,800/mi per def tank
Normal Driving: 5,800-8,100mi
Steady highway(unloaded): 8,100-9,700mi

When towing heavy I use one gallon of DEF in about 800 miles. My 2016 seems to use more DEF than my 2011 did.