There was a really good article in the FSM "How to Paint and Weather Scale Models" - released last year. I believe Matthew Usher wrote it (apologies to the author if i'm wrong).
To do it, you really need a wheel that has a rim/groove differentiating the wheel and the tyre.
Basically, paint the rim first (normally because it's a lighter colour than the tyre). Then, get some thinner and dab it into the groove separating the rim and tyre. It'll draw around via capillary action. Then dab some paint immediately onto this the thinner - it will spread around the wheel via the thinner.
I've tried this method, but was not very successful, i found the thinner dries too quickly.
So, what i do is get some very very thin paint and just dab that in the rim/groove with a 00 brush and let it be drawn around the tyre via capillary action. I need to do it several times, because the paint i use is so thin, it doesn't cover well with only one application. Once you've got that demarcation going, just paint the remainder of the tyre with a brush.
I've had great success with this on 1/72 scale aircraft.
Hope this gives you a few options to try.
cml