Here's an idea.
Mold the two halves of the tire in styrene, like the rest of the parts, with a shallow "trough" around the perimeter where the tread will go.
Mold the tread itself as a flat strip of flexible PVC.
Glue the styrene halves together and paint them. Then take the tread strips, sand the surfaces down to get that scuffed look, and wrap the tread strips around the tires. If engineered correctly it will be a friction fit in the styrene tire. Or a dot of CA will hold it in place.
That way you get both crisp sidewall detail and crisp tread detail, and no need for fancy multi-part molds.