Gaston has already answered your question well so I won't go into that any further. What I will say though is don't forget to consider the user intent here - depending on what your product actually is, people using that niche word may be looking for something different.
To use your cars vs automobiles example in a research context, you might find for example that people searching for "history of cars" are searching for recent history, whereas users searching for "history of automobiles" may be looking for info dating right back to the very first automobile.
Perhaps a poor example of my point but hopefully this illustrates what I'm getting at. While the two may technically be the same, sometimes we use variations of a word with a different intent. Your keyword focus should always be around this intent rather than just the words themselves.
I hope that makes sense!
