I'm glad that throughout this challenge, you came to realize that the focus really isn't on hyper precision, but rather just learning how to establish the kinds of relationships between our marks that will be interpreted as realistic by the viewer. There is definitely an allowable margin of error for that, and there are certain kinds of issues that may be more likely to undermine a viewer's suspension of disbelief that deserve our attention more than simple things like getting the degree of our ellipses just right.

Anyway, all in all you've done a pretty great job throughout this challenge. Your linework is especially confident and pleasing, although I noticed a few issues that I will point out.

First and foremost, you forgot that in this course, we don't get into shading for shading's own sake, and really don't bother incorporating form shading into our drawings at all, as discussed back in lesson 2. When we use hatching, it is generally because our focus is on capturing the shading of a particular form. It doesn't serve any alternative purpose - doesn't imply the presence of textural forms, for example. And so, as a rule, avoid using hatching throughout this course, outside of the few cases we use it early on, like how we distinguish a front-facing face in the box challenge.

Secondly, when it comes to capturing and implying the texture of your tire treads, I noticed that there were definitely some places where you tried to rely exclusively on shadow shapes rather than outlines. For example, #8 and #12. In many others, however, there's definitely far more attention paid to constructing each tread form with full outlines (like #18). In between, however, there are examples like #20 where you've outlined your textural forms, then used filled areas of black to help distinguish the different planes of your forms.

In the future, always remember that the filled areas of solid black are cast onto neighbouring surfaces, not onto the form itself. As you can see here, leaving the silhouette of the textural form empty and instead casting that shadow on the surfaces around it will allow the shadow to convey to the viewer how the textural form relates to those neighbouring surfaces. This is essentially how we can get the most out of these textural forms.

You can still use outlines where the textural form breaks past the silhouette of the overall constructed form itself (that is, the cylindrical tire/wheel), but don't outline the textural form beyond that. As a whole you were definitely thinking about how you were approaching tackling these textures, so that's great to see. Hopefully this helps clarify what you should be going for.

Aside from that, your wheels have come out really well. The constructions are solid, you've captured the more minor aspects of the rims/spokes really nicely, and you've been eager to pin down all kinds of details and complexities without undermining the core structure of each wheel. I'll go ahead and mark this challenge as complete.