have you read the documentation?

i'm going to summarize what i remember, but i encourage you to go read for yourself:

basically, setting the tags on your post are what indicates subgrouping and lets admins know how much interest there is in a particular tag, so they can consider whether its worth making a new group.

Rather than cars/motorbikes, perhaps a wider gamit of transport might be better?

Tags are what you want! Groups on Tildes aren't isomorphic to subreddits - topics can be moved across groups if needed, and the tag structure lets you narrow down the scope of your search. You don't necessarily need a ~cars - if you were posting, say, a video about new tech in cars, you could post it in ~tech tagged cars. Then, anyone could see or bookmark a search for the cars tag to find your post and any others across Tildes.