Excessive DEF usage towing - 2023 LZ0

[Sobo21]

Towed my boat which should be around 6k pounds.  Within 1 tank of fuel (I got crappy mileage of about 11 mpg for some reason) the DEF went from 3/4 to empty in this one tank.  I got a def warning saying I had 1,000 miles, then within a 100 miles a new warning say 256 miles.  Within 50 more miles it went to critically empty and starting counting down saying I had 500 miles until speed restricted.  It was 1am, but I was able to find a place that had def; but as quickly as the def counter went down I wasn’t sure I could trust the last 500 mile counter.

Anyone know what could cause def to be used so quickly?  As a side note I towed the boat today 75 miles, got 14 mpg and the def gauge didn’t move.  Makes no sense.

[redwngr]

I've run the def level down to the 500 mile warning (intentionally) on more than one truck. 

(one truck with segmented indicator and the other had the system that displays 'def level ok' when above 30% remainning and gives a % when below 30%)

 

Anyway, both use the 500 miles to speed limit escalation.  Both seem to be a mileage counter rather than relying on the level indicator.

(on both the 500 mile message counted down one mile for each mile driven)

 

Seems like the def level indicator is also trying to do some sort of range estimate -- so is wrong when the rate of usage changes drastically.  Usage should be in ratio to the fuel usage, so if changing from daily driver duties to towing the range estimate changes quickly.

 

I've never taken on to the 65 mph speed limit.

In the areas I was running, 65mph wouldn't have been the end of the world.

[Sobo21]

Redwngr, Thanks for the clarification on the 500 mile countdown being real miles.  I will have to see if the gauge is off so much and be more prepared when towing longer distances.