So, it's been a while since my question and, even though I got no help or suggestions, I decided to spend some hours and explore all the possibilities I could by myself with trial and error method, since I couldn't find anything about this argument on the net either. Basically, you can easily find tutorials about how to apply a square texture, being it leather, cloth, wood or other stuff to a seat, a dashboard, etc. but you can't find anything about how to use UV textures with decals for those objects where plain texture isn't good because of the complex shapes you must apply the decals to, which can have lots of curves and a sudden change of thickness in the object that deforms everything.
In the end, the method that seemed to work better is to use UV texture in planar mode. I got almost the result I wanted, the only things I could improve is that I wanted to cast the project only 180° instead of 360°, since now it's repeated on all the rim instead of just half, and I didn't want the projection to be visible also from outside the rim, but you can't seem to be able to choose a projection depth like textures in planar mode.
There are some things which I really couldn't understand about UV texture, so if someone can give me answers, I would appreciate it a lot: why you don't have a "manipulate" button inside the material panel like for "planar" mode? But if you go on "texture editor" you have the manipulator... So what's the sense of this? Also, another thing I can't understand about texture editor is why you can only rotate your texture on one axle?
Also, if someone has experience with applying decals to difficult shapes like mudguards or also the stickers you usually have on the sides of wheels/rims and where you must use UV texture for not having deformed text, I would really like to know what your workflow is for this particular kind of task.
P.S. I tryed also to cast a texture onto a cube, but with no luck at all... I really would like to know if there's an easy way to cast a decal onto a cube... I just got my decal splitted on random faces.
