Thr fibometer has its uses, once you get used to the numbers you normally see, you can spot any unusual trends which might indicate an issue, such as a binding brake.
I'm going to throw a cat amongst the pigeons now...
The MPG readouts are only an approximation, most cars appear to over-egg the mpg to some degree.
On the other hand, where do you get your mileage figures to calculate brim to brim figures, or punch into Fuelly? Odometer mileages are derived from the speedometer signal, and speedometers are all calibrated to over-indicate to accommodate tyre sizes, tyre wear, manufacturing tolerances, etc. - by up to 10%. That means that odometers also over-indicate, and if you are using the odometer readings in any calculation, they will be out too.
edit: I had to edit the last bit out, I think I had a bit of a brainstorm