I recently got this truck and I have reset the fuel economy recently. Maybe that has something to do with it.
My problem is that my Mile to Empty numbers decrease faster than the miles I travel. I went to the store about 5 miles from the house and I somehow lost 30 miles of range. Drove 20 miles and lost 40 miles. I've never seen that much fluctuations with other cars I had, including Fords.

 

In the F550 just like the F150 the Miles to empty goes off of the actual miles to gallon. I have fueled up and reset while unloaded and get 11 or 12 MPG and have 500 miles to empty get to where I am going and get loaded and drop to 8 MPG and drive 45 miles and loose 120 mile on the miles to empty. It goes the other way also have left with 8 MPG 450 Miles To Empty and gotten to where I with at 9 and 510 to go. Closer it is to the reset the faster it changes.

 

That is simply a gimmicky "driving aid" and should never, ever, under any circumstances be considered for the 'final' authority on fuel range. As noted, it is simply a calculation based on the snapshot in time of the current fuel consumption, which of course can vary 5 - 10 mpg or more depending on circumstances. If for example you'd been cruising the highway and getting 20 mpg, and you have 5 gallons of fuel left, the display would read "100 miles to empty". But now say you're driving in town, merging into traffic, and only getting 10 mpg, your "miles to empty" just went from 100 to 50. But get back on the highway again and it will change all over again.

 

The fuel gauge is the one you should be watching & not worrying what the miles to empty reading is. As stated above the mile to empty is just a "gimmicky" reading.

 

If it's a "new to you" used truck, have you checked the correctness of the odometer/speedometer. If a previous owner put on a difference size tire than stock, and not set that new tire size in the PCM, all mileage reading and calculation are off.

 

Miles to empty feature is the most useless feature on vehicles. It means nothing, ignore it. All sorts of things will cause you to get false readings. It makes those calculations based on "estimates" of how much gas is in the tank and the fuel mileage you got previously, that's the miles behind you. It has no idea what the fuel mileage is going to be as you move forward and that is the important part.

I've gotten as little as 4 mpg for short distances and nearly 40 mpg at times for short distances. I've seen my DTE readout go up when driving under ideal conditions and getting much better fuel mileage than normal. I've had DTE drop by 20 miles while stopped and idling on the interstate for an hour while a traffic accident was cleared.

I've had DTE go to 0 miles when driving up, or down steep roads even though I had 1/2 tank in a 36 gallon tank. On steep hills the fuel goes to one end of the tank and you get false readings.

My 2014 has a "gallons used" readout. I don't think newer trucks have that feature. That is what I watch. In the 141,000 miles I've drive the truck it has never been off by more than 1 gallon. Usually within 1/2 gallon when I fill up. I know I have a 36 gallon tank and I can do the math in my head to figure out how much farther I can drive more accurately than the DTE display.

 

The area I live and drive around has a lot of terrains, no matter which direction you go, you're either going up a hill or down a hill. Highways have steeper longer grades but most of the back roads are flatter than interstate but much slower. It seems I get better mileage in slower flatter roads than taking the interstate to go to work.

I see that with the new faster calculation, this truck uses than any other car, it makes it much more difficult for it to figure out my MPG.
I'm avg about 15.5MPG. When the weather is nice I plan to change O2's, Spark Plugs, etc.

 

Calculate for real after filling up, don't trust the computer. I sorta use the computer, but only in comparison to itself.

 

It's called a lie o meter for a reason...

 

Mine will do the same. I've parked it with 50 miles to empty, started it up the next morning and only had 40. Just idling at a red light will drop off 5 or more miles. Accelerating fairly aggressively onto the highway will also cost me 10 miles of "range." But then coasting down a long off ramp will gain me back 7 miles. It's definitely inaccurate, but it seems to err on the side of caution so you don't run out of fuel.

I've (out of curiosity) driven it until it said 0 miles to empty before filling it up (had a 3 gallon jug in the bed), but didn't run out, and when I filled up it only took 32 gallons, but I have a 36 gallon tank. Go figure.

For comparison, back in the day I had a 2001 Chrysler 300M, and when that things DTE hit zero, you were literally out of gas. I was driving down the freeway one day and was closely watching the DTE reading, watched it change from 1 to 0, and about 1/4 mile later I was out of fuel and coasting to a gas station. Almost made it too. That was the only car out of the 30+ vehicles I've owned that was actually accurate with the DTE.