No factual answer likely… IMHO
I went by a local government’s garage today, and saw that they had a Toyota Prius hybrid car. I tried to say that I liked the fact that the government was buying – Priuses? Prii? Priui?
How do you form the plural for Prius?
There are 131,000 Google hits for “Priuses” (and a few for “Prius envy” as well).
ETA: “Oh, what a feeling.”
ETA: Toyota’s website uses “Priuses.”
Where did you find it? I was looking through their press releases and could only find singular references (all pluralization was to “units” or “vehicles”).
I know that for Lexus, their official policy is to never pluralize the name.
Priuses. Prius is a Latin word, but it’s not a noun, so you wouldn’t normally pluralize it, and you don’t have to ame the effort to sound educated and pluralize it in Latin.
It’s a name, not a scientific term – Priuses.
Priodes
Cal, I can never tell when you’re serious, or if you’re making a joke.
Gee, I woulda thought that one was obvious. And adding any more words would have spoiled it.
Yes, many more wordses would have.
And when there are a lot of those cars, you can pluralize that: Priodeses.
While Toyota may use Priuses on their website, if you visit Prius owners sites/forums (fora!) you’ll see Prii used quite a bit. I’m not saying it’s “official” but it’s fairly common.
Well, but common doesn’t mean correct. I spell my own name correctly, but many other people spell it wrong. Many people write “penii,” which is also incorrect.
Good lord! How’s your name actually spelled?
I never said it was correct, just what I’ve seen in common usage. I’m not sure this has ended up in the OED yet for goodness sake. 
How to Google a site for a word:
“priuses site:toyota.com”
Returns 28 hits including:
Maybe they’re not misspelling your name. Maybe they’re trying to tell you something.
You sly trans Casanova, you.
Hey, you work with what you’ve got!
“Get me a Prius, and, while you’re at it, get me another.”
And if they got lost, they can go on a Priodyssey.