It can be a "lie-o-meter" if you don't refill the same way at each gas stop. I find mine to be very close and sometimes if not dead on to the gallons used.
On this last trip with the pic showing the mpg when I top-off the gas tank, it took exactly 30.0 gallons, the trip meter read 29.9 gallons used.
I refill at the pumps will the auto-shut off, after the 1st kick-off I repeat twice more at the same setting on the gas nozzle, then put the gas nozzle back in the gas pump. I don't manually add more gas to round up to the next whole dollar or to the next full gallon, this is where I believe the lie-o-meter has issues with.

I prefer to top-off at 1/4 of a tank but sometimes that can't be done. I do know with my F350's 35 gallon tank that when the low fuel light kicks on I have 8 gallons of gas left in the gas tank, miles to empty read-out is also very close to what I really have as well.

I knew this up front on the mpg with the 6.2L w/4.3 gears while towing, these mpg numbers are fine with me.
I don't use the F350 for a every day vehicle but the last time I drove it for a couple of hundred miles empty I avg 12.2 @ 75mph.

 

I just got my truck and trailer. Only used it once for a 200 mile round trip but could only use the lie o meter because the EVAP was clogged and couldn't get a good judge on a fill up. But at 60 I got over 10. At 75 8.7.

 

I was finally able to manually calculate my mileage. Up until now I had a clogged EVAP and couldn't let the pump do the work. At the same pump at the same station once it clicks I let it click one more time. I don't round up to the nearest dollar. Lie O Meter said 10.1. Calculated 8.3. I put in several gallons more than what the trip said I used. This mpg would be suburban city driving (live 45 south of Chicago). Disappointed I ran out to get spark plugs. Changing them tonight.

 

Don't freak out about one tank, and stop "topping off" the tank with that extra click. That can throw out that one tank calculation pretty easily.

 

I hope you know the 6.2L has 16 spark plugs.

 

What is quess?

 

Yes I do. Took me just under three hours.

 

2015 250,6.2, 3.73 EL,
GW 10,500#, 11,000 miles in 12 months.
My speed is 63-65 always with camper.
Iowa to S.C. Iowa to Az.
12 month average 11.6 MPG.
Every mountain we can find to climb