I've been doing the same thing for over 6,500 miles now, and my numbers
are 46.89 vs 46.28. After three rather revealing tanks where it was
clear that I didn't fill up all the way, I've come to trust exclusively
the MFDs numbers.

John1701 from most of the Prius forums challenged my conclusion that
the computer was just as accurate and suggested I look at his 120k+
miles of data. I pulled his data into my spreadsheets and found that
the numbers were within 2mpg of each other, too!

I track both, but I trust the MFD.

I'm in the Boston area.  I bought a 2006 Prius on 7-8-06.  I've been
comparing computer vs acutual at each fill up.  To date there is only a
2 mile variance and an actual milage of 49 mpg.

I traded in my 2003 Honda Hybrid that only got around 42/43 mpg.  It
never made it to 49 mpg.  Very happy with the better milage !

If you stop pumping gas once it clicks off you will get an accurate MFD
reading. I paid good money for my MFD not to operate correctly. I only
wish I had calculated MPG each time I filled up the tank before I had a
Prius.

I have a 2004 Prius that I bought used in 2005 after it have been in a
accident.  Seemed that when they reset the AC Compressor they put to much
134 Feon in it and caused the engine/motor to run strange.  The Insurance
company then ended up paying of the vehicle for the original owner and I was
able to get the car for about half the price.  The Dealer Ship in my area
found the problem in minutes and that was all that was wrong with it.
Having only about 7K miles at the time it seemed like new.

As for the mileage, around town with stop and go, with top speeds of
35-50mph.  I usually get 50-54miles per gallon.  On the high at 50-60
cruising it's more about 50miles per gallon.  Finally at 70-75mph it more
near 45 miles per gallon.  I currently have about 30K on the vehicle, and as
some of the others, keep track of the gallons both as indicated on the
readout and actual figured from the consumption of fuel.  I also use a GPS
to verify the mileage.  Which overall is fairly close on the mileage, but
some times off on the speed by usually the car indicating slightly faster by
1-1.5mph.

It may be a subject of discussion, however I found that accelerating at a
more moderate speed then just slowly getting up to speed often gets better
results.  I believe this has the Motor helping out more then the engine.
Seems like when the engine is running with out the motor drive the display
stays in the 20mpg.  However at the moderated pace it also stays in the
20mpg, a little lower maybe.  But I'm up to speed quicker and back at the
plus 50mpg.

Really enjoy reading the comment of this group.

Regards Don

To follow up:
I have now filled up my Prius seven times and driven 2722 miles.  My
calculated fuel economy since the car was purchased (with 1 mile on the
odometer) is 51.212 mpg and the compuer average for the same period is
51.674 mpg.  With less than .5 mpg difference between the computer and
my calculations, and given that the odometer only registers in whole
miles (I don't use a trip odo for this) and the computer only shows
economy in tenths of a mile per gallon, this seems remarkably accurate
to me.  On any given fill-up the computer may differ from my
calculations by as much as 2 mpg, but I attribute that to differences
in fill levels, the bladder, and other mechanical variances.
If you would like to see my spreadsheet, it is available here:
http://spreadsheets.google.com/ccc?key=pddd-ELVS2pr_aG6WEewyaA.  This
is my first use of Google's new online spreadsheet.  Some formatting
didn't transfer exactly correctly from Excel, which is why you sill see
some cell borders that don't look right.  Feel free to steal my
spreadsheet and adapt it for your own use.

Dave