For wheel C meshes, I'll usually take the center row of polys of the wheel and duplicate them. I'll then cap the sides and scale the width to match the width of the tire. I don't try to match the curvature of the tread because you'll never notice that it's off in the sim. To be honest, I've got a few models up that don't even have a collision mesh at all and I can't tell any ill affects from it. They were my earlier models so the newer RF versions may reflect otherwise.