DEF page detail taken from 2023 3.0 manual (the 6.6 has a larger tank)
The 2023 LZO tank holds 5.4 gallons, so when like the last line (when it will take 4 gallons) it still has 1.4 gallons in tank.
Daily driving that will be +/-1500 miles til empty on mine. (And then the 500 miles to speed restriction begin)
I'll need fuel twice to drive 1500 miles -- and where there is fuel there is def
(ya, maybe extremely isolate areas there may be fuel and no def, but if I'm going there then part of my preparation is filling the def tank)
Lack of accuracy example:
Drove 2000 miles since DEF tank was completely filled.
Decide to check the DEF page. (intentionally not calling it a gauge)
DEF page is showing top 4 segments off.
But -- perhaps the 4th segment just went off, or maybe the 5th segment is about to go off -- so somewhere around 1.5 to 2 to 2.5 gallons can be added.
2000 miles / 2.5 gal --- 800 miles per gallon
2000 miles / 2 gal --- 1000 miles per gallon
2000 miles / 1.5 gal ---- 1333 miles per gallon
So, using just the def page, we really have no idea about the def usage. (which is why I don't call it a gauge...)
1333 is way way more than 800....
Otoh, I'm not sure all this really matters.
If not trying do detailed tracking of DEF mileage just wait for the 1000 mile warning and dump in a 2.5 gallon jug.
I'm doing it for '****** and giggles'.