This model started life as an experiment in using bump maps to model tyre
tread, and grew from there.
The tyre texture was drawn at low resolution in an arts package, then blown
up and blurred. The edges and flat of the tyre are separate textures to enable
scaling of the width without affecting the sides.
The dirt texture was produced in Photoshop, all the parts that were to be
dirty were glued together, then the texture applied, then the objects were
split up to be glued into groups for movement and animation. This was
to apply the texture uniformly where it was needed.