- I am seeing the same exact thing you are: After driving the DTE down to zero two-out-of-the-last-three fills, I have never gotten more than 34.5 gallons on any fill.
The question now becomes, how much of those 5.5 "reserve" gallons is really accessible/useable?
I would be interested in the results of your siphoning experiment if you find there is really 5.5 gallons left in the tank, because if so then I wonder if:
1.) The DTE is just off
2.) The Titan XXL tank is deeper than the stock tank and the factory sending unit/suction pickup is too short now and can't "reach" all of those 5.5 reserve gallons
If the latter, then I will tend to agree with you:
=> If the sending unit/pickup is not replaced, then I would hope for the price we paid that Titan designed the tank to have the same pickup height as the factory tank
=> I don't really want to test how far past 0 DTE miles we can go, as I don't think you ever want to run out of fuel with this or any other diesel engine
The new XXL tank is clearly ~3" deeper (why the factory shield can't go back on) but I didn't do my install, so I don't know if the hole where the sending unit goes is offset that same depth.
- what is your max gallon fill on the larger 48 gallon XXL tank for the 6.5' beds?