-
I think it's a proper noun. But, you can choose your gender, as it's not a Latin word. So, we're just making it up. It will depend on how you refer to your car. If you refer to it as a woman, then you're probably stuffed, unless someone can tell me that there are feminine words out there with an -us ending.
-
unless someone can tell me that there are feminine words out there with an -us ending.
The fourth declension is a group of nouns consisting of mostly masculine words such as fluctus, fluctūs m. ('wave') and portus, portūs m. ('port') with a few feminine exceptions, including manus, manūs f. ('hand').
https://en.wikipedia.org/wiki/Latin_declension#Fourth_declension_(u_stems)
Isn't it complicated by the fact that 'prius' in Latin is an adjective, and so you'd have to know the gender of the associated noun before deciding if its masculine, feminine or neuter? I would've thought the plural would be 'priores'.