Well the ultimage authority - googlefight.com - says it's Prii (Prii =
324,000, Priuses = 96,400):
<http://www.googlefight.com/index.php?lang=en_GB&word1=priuses&word2=prii>.
;-)

But I'm with you on this, it should be Priuses. Prii peeves me almost as
much as faux Latin plurals of virus. (Although neither drives me to
apoplexy as much as the dreaded /chaise lounge/.)

Paul

Hi, Diako!
You made me chuckle!  I have been known to get un a high horse about
language occassionally myself, so I know how you feel.  My particular
peeve is the constant attempt to invent new words and misuse existing
words in order to keep from offending overly sensitive groups.  For
example, using "gender" to refer to one's sex.  Gender USED to refer
only to the property of words (masculine, feminine or neuter gender).
Now, dictionaries have caved in to the tyranny of common misuse and
have "gender" as a synonym for sex.  Not me.  Words have gender; people
have sex (if they are lucky! <g>).
Nonetheless, when speaking informally I hold to the idea that the
purpose of language is to communicate and sometimes that is best done
with less-than-perfect grammar and word usage.  In this case, I happen
to find "Prii" cute and fun.  So, despite your thought-provoking (and
probably correct) comments, I'll probably continue to speak of our two
prii.  You, of course, are free to be offended. <g>
And, yes, we do have our second Prius.  It is a 2006 white package 8 -
had 1 mile on the odometer when we picked it up.  It sits next to my
wife's blue 2006 package 4 in the garage and, together, they make quite
a handsome couple.  I have suggested we ought to swap the doors to make
our Prii (Oooo! There's that nasty made-up word again! Will the horror
never stop?) stand out from the growing crowd, but my wife put the
kibosh on that idea...
I got 51.1 mpg on my first tank of gas (484 miles) per the computer
readout.  I have started an Excel spreadsheet to keep track long-term
and see how reality compares to the computer.  So far, my first fill up
exemplifies the problem with calculating fuel economy on just one tank.
Even though I reasonably believed I had burned at least 9.5 gallons of
gas and the last bar was blinking (leading me to thing it was probably
even more), I was only able to put 7.646 galloons in the tank - giving
me a calculated fuel economy of ove r63 mpg!  Unfortunately, I dropped
the first bar after only 28 miles, when my experience with the other
Prius and the first tank on this one tell me it should take about 120
miles.  In other words, for whatever reason, I didn't get the tank full
by a good two gallons.  Oh, well.  It will all work out eventually on
the spreadsheet...