To answer your question first, Drawabox actually doesn't deal too much with proportion. This is actually the sort of thing you'll see more frequently in the more "traditional" drawing books - drawing from the right side of the brain, and others that focus on more pure observation. Most of these kinds of books focus on pencil (although I'm sure they work fine with other digital means). Honestly I think the best way to practice that kind of observation is just doing more studies. Don't restrict yourself just to ink, but instead do both long and shorter studies, rather than focusing on just one kind. They both have value, and while you're going to nail down your proportions earlier on in the process, there's a lot more to be gained from photo studies than that. Many years ago when I was just getting serious about drawing (admittedly after many years of doing it for fun), I would do daily 3 hour digital painting photo studies. After a month straight of that, I felt my observational skills had improved quite a bit.

Now, moving onto your wheels, overall you've done a pretty good job but I did notice some things that stand out as being a little strange. I'm fairly certain you're working with an ellipse guide in most of these, as the ellipses themselves are fine, but some of them (like number 11) tend to have these weird protrusions - perhaps a mistake with the central ellipse, where it is actually narrower than the two outer ellipses resulting in a very strange alignment. This is something that comes up often on this particular page (11-16). My only other thought is that perhaps your ellipse guide is very limited, and you're stuck between two specific degrees.

This particular issue aside, I am pleased with the rest of your constructions. You're quite fastidious when it comes to building out the complexity of your rims, and you clearly take great care with most of your tire treads. There were some cases, like 21, where you ended up focusing too much on the tread as a pattern of lines cross-crossing across the tire's surface. Remember that these are still textures, and all textures are made up of cast shadows, not the lines themselves. And therefore you do need to at least be thinking in terms of the forms that are present along the given surface, which cast those textures.

One kind of tire tread I specifically keep an eye out for only came up once in your set - number 13. This is one where the tread is a little more chunky, with individual pieces that stick out. In this case, you outlined those sections of tread explicitly, which suggests that you did indeed forget about some of the principles of capturing texture from lesson 2, specifically the section on implicit drawing techniques. That is, the importance of capturing textures purely through cast shadows, and not relying on outlines at all.

So! Be sure to review the sections I linked above. Aside from that, you're doing quite well, though I do hope that strangeness with the center ellipses gets sorted out. Based on how odd it seems, I'm pretty strong in my assumption that it was an issue with the tools, rather than a misunderstanding.

As such, I'll go ahead and mark this lesson as complete.