Brian, on the first page of this thread you got some solid advice on making silicone molds and casting your tires in resin. If I may expand on that a little, I'd recommend Alumilite's Alumires RC-3 Black resin for your casting. It makes for a nice, realistic black finish right out of the mold. It is very opaque, so no light transfers through making your castings look "toyish." Simple 1-to-1 mixing ratio. Here's some small parts I cranked out with simple squish molds, except the Delco battery, which is a one-piece. I'll file the bottom of it flat to get rid of the excess, and the others have thin flash that an X-acto will clean right up. Most of it will actually flake right off it's so thin. My point in showing this is to show just how opaque the black resin is. You can find it here: https://store.makelure.com/store/p/1053-AlumiRes-RC-3-Black.aspx