Does anyone remember enough Latin to know if Prii is a grammatically correct way to pluralize Prius? I think it is not, but the grammar rules of Latin are very complicated. Many people think that a Latin noun ends in -us for singular and -i for plural, but that is not true. If Prius is a second declension Latin noun, then its spelling is like so: Case Singular Plural nominative PriÂ·us PriÂ·ī genitive PriÂ·ī PriÂ·ōrum dative PriÂ·ō PriÂ·īs accusative PriÂ·um PriÂ·ōs ablative PriÂ·ō PriÂ·īs vocative PriÂ·e PriÂ·ī locative PriÂ·ī PriÂ·īs But if this is so, then your car is a "Prius" only when it is the subject of a sentence: "Prius is the world's best car." (singular) "Prii are the best cars in the world." (plural) But in Genitive case: "I am a Prii fan". (singular) "I am a Priis fan" (plural) The most common use of Prius is in the Dative case: "Let's get in the Prio and go to the mall." (singular) "Let's log into PriusChat and talk about our Priis." (plural) "Let's go to the mall in the Prium." (singular accusative) "Dude, where's my Prii?" You get the idea. You don't pluralize something in Latin by just taking an -i onto it.