Originally Posted by rsilvers
Surely the Ford internally knows the instantaneous result, even if it does not display it. My way (which is actually the Honda and Toyota way as well) does not start out higher than reality and drop. It just starts out with an instantaneous result and then keeps a running average.
The car is constantly monitoring fuel flow, as well as air flow, but I couldn't tell you without knowing how they calculate fuel economy how it actually works.
Ford deliberately made it an average mpg display in the trip computer, there is no code for it to calculate an instant mpg on Focus Models sold in North America. If you're looking for instant feedback you'd need to tap into the obd II port with a scan tool of some sort, but even this is not necessarily accurate if its making an interpretation of the same data as the trip computer/ car itself.
It appears that while it might start off nearer to where it was before it tends to have to think about it first starting from a lower number as its sample size is too small to give an accurate reading until eventually stabilizing somewhere near the steady state condition.
The average is constantly shifting through out the entire trip and or tank due to various driving conditions and it does take them into account, not always successfully however. Sometimes the end result is higher or lower than that shown by the mi/gallon method, but which is more accurate is hard to tell in small scale 1-2 tank measurements, you need to average both over longer periods of time, but this is off topic to the discussion of the DCT.