I'm honestly pretty surprised that you didn't use ellipse guides on most of these - your control of your ellipses is honestly stronger than the majority of people who hit this stage. Generally speaking, ellipses are among the harder of the basic mechanical skills to get a firm grasp of (especially in ink), and so I push students to get ellipse guides mainly so they don't get pushed off track when their ellipses don't come out entirely correctly. Yours however appear to mostly be strong enough not to get in the way. That's certainly a good sign.

As a whole your constructions are looking good, and while the hubcaps are indeed simplistic, you clearly demonstrate the capacity to capture them properly. In your last few pages though, I did notice a tendency to fill in the negative spaces between them with solid black. This is... technically a legitimate option, given that the depth of the wheel would likely be filled in shadow, but I'd definitely be hesitant with encouraging that as the default. In general, filled black shapes should always be reserved for cast shadows - and in order to determine what areas should be in shadow, we have to consider the forms that cast them, and the surfaces upon which they're cast. In this case it makes sense for that whole area to be black, but it won't always be that way.

Now, on the topic of cast shadows, arguably with all tire treads being textures, it's important to remember back to lesson 2's section on that topic and how we ought to go about communicating that kind of information. When the treads themselves are relatively shallow, a line can often be adequate to capture that kind of depth, but treating even the lines as though they're shadows means not necessarily just outlining each groove on both sides. After all, shadows are biased to one side, so you might end up with a shadow on one but not the other.

Furthermore, with chunkier treads like 17, there we get even deeper into the importance of thinking in terms of shadow shapes (which would be drawn using this kind of two step process rather than just as individual strokes). As discussed here on the topic of implicit drawing techniques, remember that textures ultimately shouldn't be drawn by outlining textural forms, or capturing any of their internal detail. Meaning all the lines you used to effectively "construct" each chunky tread is too much, and results in a much noisier visual in the end. Instead, focusing on the shadows cast by each chunk allows us to let them bleed together where necessary, or to let those marks grow sparse where we don't want too much ink. Where lines force us to capture each one, shadow shapes give us far more control over the density of those marks.

So! Be sure to keep that in mind, this far away from Lesson 2 its principles are still in full swing. Aside from that, keep up the great work. I'll go ahead and mark this challenge as complete.