About Tires and Rims....

There's a couple of different ways to do it Hector

1) - a very steady hand with your tongue sticking out of the corner of your mouth while holding your breath.

2) - go to a local arts and crafts supply store and pick up a circle template. Find the size that closest matches the outside of the rim and mask off the rest of the template. Paint the entire assembly black for the tires. After it dries take your circle template and line it up with the rim and spray your rim your vehicle colour.

You can also get a simple protractor from your local WalMart or the like, chuck you smaller handled Xacto with a #11 blade in it and measure the diameter of the wheel you want covered. Roll out some masking tape on a flat low-tack surface, trace out as many tape masks as you need, apply them to the wheels and rock on. This works with virtually any scale, except maybe 1/144 a/c wheel-too darned small to see!!!!!!!!!!!!! :)

Hector,
With most humble apologies to Robert and Keyworth, you don't need a mask, template, or any other of that
other stuff . They don't know what they're talking about (insert evil laugh here) Just kidding, guys....Actually, we all have our own techniques....this is mine:
Spray the resin wheel flat black...the whole thing. With the airbrush on a fine setting, take your base color, and "dot-in" the color, as if trying to paint the rim polka-dotted. Allow the dots to overlap some so that the black barely shows through. This gives you the first hint at faded paint. Don't be too concerned about minor overspray onto the tire; a wash will take care of it. Take a black, or other dark wash and run it around the rim wher the tire joins it. That should adequately any remove any overspray. Drybrush and you're finished!
Good luck with whatever technique you decide to use.
Gip Winecoff