I reset my trip meters.....but leave the avg MPG as an ongoing total. My calculations have me slightly higher than the on board computer. The difference is not substantial (within a half mile or so).

 

We have kept records since new and always reset the computer and are getting 28mpg in and AWD 2008. Since this is true MPG and in the middle of the federal EPA est. I think that it is fine. I get better MPG's then the wife but since it is her truck I can live with what she is getting.

I have found the computer is also about 1-2 MPG's optimistic. I expect the average to drop in the summer due to AC use. In the winter she lives with the seat warmer on and the heater blasting too.

 

The on-board computer MPG seems to be 1-2 MPG high on all model years.
But, the computer is at least consistent.

You cannot fill your car the same way twice, no matter how much you try.
There is more than 3 gallons of air pocket inside your tank, and you fill that to a greater or lesser degree each time. It is a complex shape, not like a milk bottle, so it is hard to predict how much air is left during each fill-up.

Thus, most long-time owners ( myself included ) trust the display, and subtract 1 or 2 MPG mentally. Mine has always been too high in 3 years, 65,000 miles.

The on-board only has a finite memory, so you cannot keep a "lifetime" average without using a database ( like on here ) or your own pen and paper records.

The memory discards old data when the memory is full.
The memory is full in about 30 hours of key-on time.
But Ford has not released the actual technical info for this, and they probably don't know themselves because some computer expert from India or China who designed the thing 8 years ago probably no longer works for the company!