Although the miles to empty is never right (and I mean never) the MPG part seems to have gotten more accurate as each new model year comes out.
The trip computer in my 04 F150 was always way off (I reset it and the trip odo every time I got gas just like I do now). It could be off by as much as 2 mpg, and that could either be + or minus depending on if I was on a trip versus a bunch of city driving.
The trip computer in my wife's 2012 5.0 is usually 0.5-0.6 mpg on the pessimistic side, so if it says 17.2 mpg she would get 17.7-17.8 when figured manually.
The trip computer in my 2014 5.0 is usually spot on, never more than 0.1 mpg off, I am actually quite pleased with it. That said, the miles to empty are never right when I check that side of it up against the current MPG reading.
Years ago in the F150 I had figured out that the miles to empty is being based off of the size of the tank minus 2 gallons as a reserve. The method I used to come to the conclusion was that I would get gas as soon as my 50 miles to empty alarm would go off. Then I hand calculated my mpg for the tank, and I would always take 2 gallons less than what I should have needed at the current MPG. I guess this was Fords way of covering their butt if someone were to blame THEM for running out of gas (not that they should have needed to, but in this overly litigious society we live in now one could understand why they would).
I am not 100% sure that is the case with the Mustangs but it may very well be the case. I guess its time for me to do a "study" again lol.
Hers:2012 Mustang GT Premium,6M,Kona Blue Exterior/Charcoal Interior, 400A, HID/Security Pkg ,Comfort Pkg.
[COLOR="DimGray"][B]His: 2014 Mustang GT Premium,6M,Sterling Gray Exterior/Charcoal Interior,400A, Reverse Sensing/Security Pkg, Comfort Pkg SMR# 3635