Don't top off!
Overfilling is not a good idea on any car, but especially
so in a Prius with a bladder tank. The bladder has been known to belch
up excess gas "squeezed" into it on a slow fill, which is not good for
your now gasoline-soaked clothing, the paint on the car where the spill
was, and the large amount of unburnt hydrocarbons you just released into
the air. (Driving a 2004 Prius for 24,000 miles produces fewer smog-forming
emissions than spilling 1 cup of unleaded gasoline, so says a Toyota
brochure.)
Also, filling the filler tube full of gasoline will usually fill any
car's charcoal canister (part of the emissions/vapor recovery system),
overloading it and possibly poisoning it, such that your check engine
light will go off, which is the least of your troubles...
Some articles:
http://www.patgosscarworld.com/donttop.htm
http://www.phillyburbs.com/pb-dyn/news/ ... 23947.html
http://enviro2.blr.com/display.cfm/id/48985
http://www.sbcapcd.org/edu/dont-top-off.htm
http://www.weights.az.gov/Shoppers/gasstations.htm
and as for figuring out your fuel economy - best bet is to use the same gas station/pump all the time (same shutoff), and to average several tanks rather than just one particular tank at a time. It's not uncommon to get a really low fuel economy tank followed by a really high fuel economy tank...