Starting with the structural aspect of the wheels, you're knocking it out of the park. Your use of your ellipse guides is every effective, and you've shown consideration both for building out each wheel with as many ellipses as are required to create that gentle "bump" through the wheel's profile which helps make the tire appear more inflated, rather than completely solid. Also, when you build out your rims/spokes, you're mindful of not only establishing the outward face of the structure, but also the side planes which goes a long way to make them appear more three dimensional and believable.

Now, the second part of the exercise - the tire tread itself - is something of a trap. Generally speaking most students tackling this challenge find themselves so far removed from the textural concepts we discussed in Lesson 2, that they forget them altogether - and it seems you are among them, at least to a point. Many of your tire textures have been handled using explicit markmaking (basically using construction, like in wheels 20 and 19). There are other cases where you've tried to work more implicitly, but you appear to putting down individual lines, rather than actually drawing shadow shapes as discussed in Lesson 2.

When it comes to employing constructional techniques, like wheel 20 for instance, it may work fine in isolation when we're just looking at a single wheel and nothing else. But when it is a part of a larger drawing, all of that linework packed into a tight space results in a lot of contrast. It becomes a focal point, drawing the viewer's eye whether you intend to or not.

Implicit markmaking however allows us to imply the fact that the textural forms are there, but only provides enough marks to suggest that they're there, and allows the viewer's brain to fill in the rest, allowing it to be as subtle or as obvious as we choose, without actually changing the nature of the texture we're conveying. Here's an example of what I mean. Based on how I choose to shift the lighting situation, I can get really deep shadows, or really small ones. You can also think of this like a sun dial, where at noon the shadows are very small, and at dawn and dusk when the sun is low on the horizon, the shadows get long.

Another point worth mentioning is how you tackle the more subtle textures where you've got grooves in your tire. These can generally pass more easily using explicit markmaking (unlike those with larger, chunkier textural forms), but there's one caveat - we can still think about those textures incorrectly, and this can influence the way in which we draw those marks in very subtle ways.

The thing is, people often end up thinking of such textures as though the grooves themselves are the textural form in question. They're not, of course, because they are void space - like holes, they're not actually solid forms of their own. Rather, the walls and floors of those grooves/holes are the textural forms which are casting and the surfaces which are receiving shadows.

Number 21 is a good example of this - you drew a criss-crossing pattern of lines, but your focus was not on thinking of how the textural forms themselves exist in 3D space, how they relate to one another, and how they cast shadows and upon which surfaces. Rather, you saw a pattern and attempted to replicate it, without understanding the 3D forms at play.

All in all, your work is still fine - I do strongly urge you to review the texture material as you reach the end of the course, but this was indeed an intentional trap meant to serve as a firm reminder that these concepts are still important. I'll still go ahead and mark this lesson as complete.