There isn't understanding as you distinguish it. Your distinction is too sharp. Imagine Roger Federer's understanding of tennis vs. a college tennis player's understanding vs. mine (not so good). There are degrees for this.
Taking your example, at a certain level, I suppose, it is possible to have a very solid understanding of an automobile, but then there are people like the guys on Car Talk - they have a sophisticated and refined level of understanding that most people will never achieve about cars - or anything else - their entire lifetime.
One thing I have heard stated is that:
"when you know something, you can use it; when you [master] something, you are used by it."
So, while this doesn't directly offer a rigorous philosophical answer to your question, I suggest that it proposes a more useful pathway to establish a place where your question can flourish. Unquestionably, there are epistemologically sound answers that philosophers have proposed to this question - I don't know them - but then answer to your question won't be found there.
Think about the paradox of it - if you were to read an answer to this from a renowned philosopher - Heidegger, Hilary Putnam, etc. - you would find yourself with a new gap between what is possible to understand about this topic, and what you already understand. In receiving your answer, you would be reintroducing the dilemma that necessitated the question.
What is possible is mastery. Mastery as I understand it is a process; a process where everything undertaken seriously becomes ripe with possibility. Because, there is no top to this mountain called knowing, everything can be looked at freshly. Everything is new and not known, and everything is a potential source of learning. Knowledge is a side effect of the pursuit of mastery.
In areas where I have any degree of mastery (there are maybe one or two), I find that almost every new thing I learn leaves me knowing considerably less than I did before I learned it. Now I know that thing (to whatever degree I do), but a whole new heretofore unseen world opens up in front of me with new things that I don't know about the world of what I've just learned.
Pragmatically, the distinctions could be drawn better than I have. Perhaps the next answer will do that.