I calculate FE by hand for comparison, however I trust the display more than my own calculations. The tank method has lots of variability. Let's list all I can think of. If you stop pumping at the first click off by the pump, there is lots of varibility there at the nozzle. Pumps are not going to consistantly click off at the same point at all pumps. So one pump may be generous, another may be sensitive. If you pump in more after the first click that will cause varibility because you will never really be able to get the same total amount of fuel in the tank at each visit, which is what is necessary for tank based calculations to work. Oh, we have the bladder which affects both nozzle auto off and our own pumping accuracy.

However, fuel economy as displayed by the car is calculated based upon fuel metered over distance traveled (in the simplest form). This is a much more accurate system than baseing it on tank averages, in which the refill on the tank can vary greatly, but the fuel metered into combustion is a value known to the vehicle along with speed, which represents distance traveled.

 

As long as the ODO is correct (mine is dead on in my 2006, I clocked it on Radar and GPS), any variation in the fuel you add eventually comes out. So the total is always going to be accurate. You add a bit less this tank, you'll add a bit more next tank. It'll all even out.