Background

There has been some speculation that the MPG at the end of a tank is lower than the MPG at the begining of the tank. This can be tested by doing a daily log. However, my bi-weekly fill-up may provide some insight:
The data does not rule out exceptionally low MPG events. If it exists, the data suggests it happens with a period of less than ~200 miles.

Here is the raw data used for this chart from the GreenHybrid.com database entry for "Prius 03": 

mi ,mpg (<=20 min),mpg (>20 min)
212,41.9,
183,,65.4
181,65.7,
82,,56.3
268,51,
200,,47.4
153,57.7,
162,,62.4
139,63.9,
243,54.2,
227,,54.3
152,55.4,
181,53.8,
176,,50.5
192,54.1,
178,61,
259,,52.1
195,46.4,
112,49.7,
140,,51.9
144,46.4,
158,,46.3
215,53.9,
123,,57.7