I recommend saving your various baked renders as separate images, and then combining them in a 2D Image Editor that supports layers (such as GIMP, Photoshop, and so on.).
Once you've combined them together in a way that you like, you can also go through and manually paint highlights, shadows, and decals onto your texture map.<pagebreak></pagebreak>
For instance, I've quickly painted a basic tread pattern onto the tire:
This doesn't look particularly realistic (as a tread pattern), but it's just an example. You can easily find real tread patterns online or paint your own.
Another thing you can do is take photographs ...