Starting with the structural aspects of your wheels, you've done a really good job here. You've included a gentle bump in the profile of the tires, which helps to make them feel more inflated, as though they'd drop with a bounce rather than a heavy thunk. The same goes for your rims - you've established both the front face as well as the side planes of the spokes, which helps to make those structures appear more solid. Your use of ellipse guides here is fantastic - and looking at the scale of those ellipses, I can see you've gotten a really solid set of ellipse guides that cannot possibly have been cheap. Good on you for that!

Moving onto the textural aspect of the challenge - that is, the tire treads which are composed of a bunch of forms arranged along the surface of a cylindrical structure - I can see that you're definitely aware here of the need to handle those tire treads using implicit markmaking, as texture, rather than using explicit markmaking and construction. Many students at this stage are so far removed from Lesson 2 that they forget about this altogether. While their results may look fine with the wheels floating separately, in isolation, but of course as soon as they are part of a larger vehicle, then those wheels become inadvertent focal points.

That said, the way in which you've tackled your implicit markmaking does have its issues. You've correctly tried to work with filled areas of solid black, but the key point is that as discussed here, the filled shapes we draw are always cast shadows - and cast shadows are always going to be shapes that are designed, and the shape itself is what establishes the spatial relationship between the form casting the shadow and the surface receiving it.

When we deal with textures with holes or grooves - like wheel 19 for instance - we can fall into the trap of perceiving the grooves themselves as being the textural form. But it's not - it's an absence of form. The real forms in question are the structures around the grooves - the walls around them and the floor at their depths. Here's a diagram that helps explain how we can think about these.

16 is another interesting circumstance - here you're very close to being spot on, aside from one issue - I think that rather than drawing the shadows those forms are meant to cast, you ended up filling in the side planes of those textural forms, which is more akin to form shading. The difference is very subtle in cases like this, but I have a diagram/explanation I made for another student that may help explain.

Here's the diagram and here's the accompanying explanation:

In the top, we've got the structural outlines for the given form - of course, since we want to work implicitly, we cannot use outlines. In the second row, we've got two options for conveying that textural form through the use of filled black shapes. On the left, they fill in the side planes, placing those shapes on the surface of the form itself, and actually filling in areas that are already enclosed and defined on the form and leaving its "top" face empty. This would be incorrect, more similar to form shading and not a cast shadow. On the right, we have an actual cast shadow - they look similar, but the key point to pay attention to is shown in the third row - it is the actual silhouette of the form itself which is implied. We've removed all of the internal edges of the form, and so while it looks kind of like the top face, but if you look more closely, it has certain subtle elements that are much more nuanced - instead of just using purely horizontal and vertical edges, we have some diagonals that come from the edges of the textural form that exist in the "depth" dimension of space (so if your horizontals were X and your verticals were Y, those diagonals come from that which exists in the Z dimension).

And that about covers it! All in all your work is still coming along great. I generally use this challenge as a 'trap', springing it on students as a reminder to reflect upon the textural concepts. You however were well aware of them, and while your application wasn't all the way there, you've shown a clear awareness of it, which is further along than most.

So! I'll go ahead and mark this challenge as complete.