Buy some Vallejo Dark Rubber. Thin it just a bit with Windex or Vallejo thinner or whatnot. Not much, just enough to give it a tiny bit of flow.
Next get a brush that tapers to a nice, fine point. You don't want one of those microscopic brushes...the one I use has about 1/4" worth of brush on it.
Dip just the tip into the paint and then wipe off some of the excess on the rim of whatever you're painting from (I use bottlecaps), then very gently, pulling the brush toward you, trace the outer circle of the wheel...there's usually a very nice dividing line. By going gently, with paint that's not too thinned, it's shockingly easy to get a nice circle. I use this method on all of my wheels of all kinds, from big 1/32 bomber tires to tiny 1/144 wheels, and of course armor.
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com