On the Consumption Screen:
MPG chart over the last 30 minutes, in 5 MPG increments, the newest 5 minutes is on the right (bars move left and "fall off" the left side of the histogram). Resets whenever you turn the Prius off.
On the Consumption Screen:
"Instantaneous" MPG, only about the last 2 seconds or so, bar on the far right side. (If you have a 2004-? Prius, you also have this as a numerical value on the Energy Screen.)
On the Consumption Screen:
At the bottom of the screen, you have an odometer and the average MPG over the miles on that odometer. The odometer can be reset by pressing the "reset" button on the screen. If you have a 2004-? Prius, there is an undocumented feature that this odometer will be reset whenever you fill your gasoline tank (except for a few rare circumstances). If you disconnect your 12v battery for a long enough period, it will also be reset.
So, if you want a per-tank MPG, you can either:
* Record your Consumption Screen odometer each time you fill your tank and reset it (or if a 2004-? Prius drive <1 mile for it to reset for you)
* Record your odometer (best to use and reset a Trip A or Trip B odometer) and the number of gallons used to refill your tank. (Variable, as you cannot refill to the same point every time...)
Sadly, there is no lifetime MPG indicator in the Prius, like can be found on the Honda hybrids. For that, you either have to NEVER reset your Consumption Screen odometer (only available on a Classic Prius), or you record all gasoline fillup data.
A per-trip MPG will depend on the length of that trip (within one tankfull, or several tankfuls), and will be some combination of above.