Personally, I would say the problem is the paint. Tamiya acrylics are hard to brush. I find that they are OK for small details, but on larger areas ( I would include tires or wheels in that) it seems like you need a lot of paint to get a decent result; I always keep the brush "loaded".
Fo my roadwheels/tires I use the cheap-o acrylics from Micheals or Hobby Lobby, the kind in the plastic squeeze type bottle. I think Apple Barrel is the brand. They cover well, wash up with water, and dry very fast.
Another option would be to paint the black tire first, then get a cirlce template or stencil, the kind that has 10-15 different sizes on it. Find the size that works for that wheel, then hold the wheel on it and paint the camo color, using the stencil as a mask.