"I likies the carses"
You don't have to be a mechanical genius. You don't have to have enclyopedic knowledge. You just like cars, and you pick them apart in ways that the standard consumer would not.
You can drive a subcompact. Or a pickup truck. You don't have to own a sports car.
TBQH, I almost use the term in a derogatory sense sometimes. Usually I'm referring to someone with zero knowledge, and no desire to attain any.
But even then, if they're paying someone else to mod their car to meet the desires they have for driving...well, I think that's still enthusiast.
Here's a good way to put it maybe:
-Large base of car knowledge. The history, the constant changes and revisions to car lines, different engines, different designers, ect. Or just a fundamental understanding of how a car functions. Some substantial mix of all that stuff.
-Ability to fix anything automotive-related. Or at least make an effort (and preferably be able to admit defeat/seek help before you destroy something).
-Love of driving, and having a far better than average competency level. This is where the ''manual' thing comes in for me. These types usually like to manipulate things themselves. Saying you like driving your cushy, fullsize, well-equipped automatic is a bit different- enjoying your mobile living room is not really the same as 'liking driving.'
Anyway- pick two of the above. Maybe one if you have an absurd strength in that area (but for that, you'd probably automatically integrate one of the others. E.g. car-fixing savant probably has a pretty good knowledge base)