'23 Lariat 6.7 ~6300 miles on the clock now.
In the first 5000 miles I did not do much towing and averaged about 18.5 mpg sometimes getting over 20 on highway trips empty. Put two 2.5gal containers of DEF in so far.

Hooked up my 10k 5th wheel and hauled it on a ~350mile trip including some mountain passes. (Love this truck BTW. Pulls amazingly)
Not surprisingly, it drank more DEF towing. If I remember right, the "Diesel Gauges" screen went from 33% to 20% on the way down and I started getting DEF warnings. Added another container at my destination. Using Walmart Supertech DEF if it matters.

Same thing on the way back. Used a significant % and had just started with the warnings by the time I got back. I filled up and bought another container of DEF but since I wasn't towing, I figured I'd let it ride.

There are two dash screens that show DEF. "Diesel Gauges" shows % and "Maintenance" shows miles left.
The % was around 32% and the miles were 200 something. Back to around 18.5mpg and the % quit dropping but the miles left kept dropping mile for mile. I would get the DEF remaining mileage warning every time I started the truck. Occasionally the % would show as low as 20% but would then show 30%-32% later.

Just yesterday, the mileage remaining ran out. Stopped at Tractor Supply with 1 mile left. When I left, I drove 1 mile and watched the DEF % go down from 20% to 10%, then 5% and count down to nothing in the last seconds. Then I got the warning that DEF was empty and I would be speed limited to 50mph at next startup.

So I pulled over and shut the truck off. Sure enough, at startup, I got a warning that DEF was empty and I would be limited to 50mph. I was. Truck would go about 52mph but that was it. Strangely, the % screen was back up to 30%.
I pulled over again and added the 2.5gal jug of DEF. When I started back up, I got the empty and 50mph warning and the % was up to 64%. In the minute or two of idling before driving off, the warning went away. The truck drove normally, went over 50mph just fine, and still shows 64%.

Does anyone have any insight as to what's going on? Obviously, the % is not just an honest gauge reading. It's manipulated by the software and it dropped from 20% to 0% in a mile.. Neither is the "miles left" reading as it seemed to be stuck on the DEF consumption rate it had from towing and continued to decrease even when the % didn't.

Working from home, the truck doesn't get driven a whole lot. It can go a week without being driven at all. Should I keep the DEF topped off? Add just 2.5 gal when I get a warning? It would be nice if we could trust the gauges but this proved to me that it's not just a straight gauge reading.

 

do a complete fill and it will be much better.

It gets down to <200 to empty, do a complete fill.

 

If I had more options for DEF at the pump, I would probably do that. Using 2.5 gal containers, I don't really want partial containers laying around.

I did find a 7-11 nearby that has it at the pump but It doesn't seem to be a high volume station and is much more expensive for diesel than the high volume QT I like to fill up at.

There's a new Buckees opening soon just north of me. Maybe they'll have pump DEF.

 

When the DEF gauge shows 1/2 tank on my 2017, I add 2 1/2 gals of DEF. No problems so far.

I also use the fill as a reminder to check my water separator.

Make sure the seal on the Walmart Super Tech has not been tampered with. There have been stories of yahoos returning the jugs with just water in them.

I also check the DEF in the jug with a refractometer just to be sure.

 

you are overthinking the partial jugs issue, just keep in the garage, with cap on. I could be wrong, but I think the 23-24 MY's have a 7.5 gallon def tank. Fill it completely. By the sounds of things, you are actually near empty, and you just put 2.5 gallons in, so you will keep getting the low DEF warnings. the guage is not real accurate either. they are not a float style sender like in the fuel tank. Its more like RV water tanks with metal sensors in the tank at 25%, 50%, 75% or some other percentages that the computer then interprets as a fuel gauge.

 

The tanks are 7.5 gallons and the DEF level sensor is not like RV tank sensors, IIRC they are ultrasonic sensors and they take a couple of minutes to read correctly once the engine is started.

Edit: BTW I have over 10K on the clock with 3000 towing. My DEF usage around town is around 450 MPG and towing a 7K TT is around 300 MPG. Your larger trailer would probably be around 200- 250 MPG, depending on how you drive. Fill the tank, just adding 2.5 gallons when it gets low doesn't help much going from 1/2 gallon in the tank to 3 gallons in the tank.

 

Thanks for correcting me. My point still stands though to the OP. Fill the tank to the top.

 

Thanks for the info. I read some accounts of people filling their tanks and the truck not recognizing it. In my case, it seems like the level indicator is working fine. It always said ~30% and immediately read 64% after adding a jug.

The interesting part is that the truck's software seemed "stuck" on it's mileage warning. Had I still been towing, I'm sure that mileage warning would have been closer to valid.

I laughed out loud when the miles to empty ran out and that part of the software basically forced the level gauge to lie. 30-20-10-5-4-3-2-1-0 in a matter of 15-20 seconds.

I understand and appreciate everyone's input about keeping the tank full and I'll do that. I do like to learn and figure out potential issues to the extent I can while I'm close to the house so that I'm less likely to be caught off guard when travelling far from home.

With gas vehicles, I would usually take a gallon or two and run it dry one time to see A. How far it would really go on "empty" and B. How much the tank actually held. Obviously not going to do that with a diesel

 

Costs just as much to fill the top half of the tank, as it does the bottom half.. why not just throw a jug in when the gauge reaches half? It will take it all, and get you back into the 85-90% full range, meaning you should get several thousand miles range
whether empty, or towing. Never get caught short, or de-rated power..

 

You NEVER want to run out of DEF, this will put it in derate and it is a royal PITA to get it out of it.

 

over 10 years with this stuff, always my practice to let it run down to <200 to empty then a complete fill.

It gets less confused that way. Use the ford filler spout to prevent overfilling if needed.

 

FWIW I stopped by the DEF pump this evening and filled it up. It took 2.396gal more or less confirming the 64% reading.
So in 6652.6 miles, I've put in 12.396gal of DEF. Works out to 536mpg.
Maintenance DEF reading says 2500 miles to empty (though in my experience it won't budge for quite a while) That would be 333mpg.

 

Kind of the point of this whole thread.... The truck TOLD me I was out of DEF and even went into 50mph restriction mode. It also said I had ~33%.
It just took me 4.896gal to fill it so that 33% in the "Diesel Gauges" screen was pretty accurate. The 0 miles to empty in the "Maintenance" screen (and the speed restriction) was not.

 

Hey neighbor! IIRC 3/18/24 that is the biggest gosh darn gas station I have ever seen

 

Something similar has happened to me when towing. You want to put 2 jugs in it once you are getting down to around 1000 miles remaining. I have never topped one off and have owned 3 6.7s.


For regular duty I just throw a jug of the walmart stuff in there. Grab it when I think of it when at walmart, not very often.