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.