I just put a large capacity (68 Gallon) fuel tank on my 2025 F350 Dully. When i say I, actually I mean I paid the Ford dealership in Gilbert Arizona to do it. The issue is that I think the calibration is off and each time I take it to Ford they say oh, no it is ok, you just need to drive it for a while. Well i have over 1400 miles on it and it still reads low. Actually recently i took it isn for a recall (cluster software update) and they said that was probably causing my issue, the dealership probably reset the tank to a 36 gallon tank, and this made send because a 36 gallon tank would read approximately 576-612 miles to empty when full and my average MPG was between 16 and 17, so i brought it back in and they said it was ok, that the tank capacity was still showing set to a 68 gallon tank, which i find strange as a 68 gallon tank for a truck averaging 16 miles per gallon should be showing should show 1088 mile to empty rather than 611. Actually averaging 17 miles per gallon on a 36 gallon tank would equal 612 mile per gallon, so is this coincidence or is the cahnge to tank capacity not taking affect?

Here is the issue: It is a 68 gallon tank, they say they updated the software to indicate it is a 68 gallon tank and the software does the rest, but when I drive the truck and I am getting an average of 15-17 miles per gallon, and the miles to empty say 75, I stop to refill the tank and I assume I will be putting in about 60 gallons. But 1) when I fill up the tank and after getting 25 gallons in the tank the fuel gauge reads FULL and 611 miles to empty, then 2) the tank takes an additional 26 gallons before the fuel shut off kicks in, indicating there was about 16-17 gallons left in the tank and should have read between 240-272 miles to empty when it read 75 miles to empty.

I had a large capacity tank on my previous SRW F350 and it reacted as you would expect, as soon as average MPG changed the estimate miles to empty updated, just not on this new truck with the new tank. This makes it very nerve racking not knowing and not having confidence on long trips to know i can trust my instruments.

Hs anyone seen this before and does anyone have any ideas what is wrong and what I can do to get the corrected?

 

You need to get OBDLink MX+ on Amazon on Black Friday, keep this adapter if you like it.
With the MX+ adapter and when using OBDLink app you have free OEM enhanced support so you can retrieve ALL (if not 99%) ford's specific DTCs, additionally you can add display from a few hundred preconfigured PIDs for Ford.
Example:
Trip Distance (miles), Trip Fuel (gal), Trip Fuel Economy (MPG), these are standard PIDs.
Fuel Level (%) is non-standard and it requires Ford enhanced support.

What is it about and why is it relevant to your situation?
All 4 PIDs shown above have its own hexadecimal code and different formula and they are not linked to each other.
The dealership might have forgotten to address all possible PIDs (by coding) you are interested in. They might have coded only Fuel Level (%) but not the other 3, or are using the wrong hexadecimal codes. It's more than just setting it to 68 gallon and forget it. It might be possible to reset adaptation to fix it, you need to ask them.

If you buy the MX+ you can verify this yourself.