@KoSprueOne: Thanks. That was my thinking: cast tires and wheels as a single piece.
@Russ: Yes, you are right; I said rubber, but really I meant vinyl. The tires themselves are nice, but I have an aversion to using anything vinyl because my experience has been that it's very difficult to get acrylic paint to adhere. I'm afraid my primer might eat them up eventually. I do have some automotive adhesion enhancer that might work. Had not considered that till just now...
@Rob: Thanks. Was not really contemplating using pressure casting; rather, just basic gravity casting.
Thanks for the tips, guys!