If you want the metallic look that I made for the pack I released, make the texture colour dark, and the specular colour fully saturated. For example, here is a colour/spec/norm combination. Note: you don't need a normal map for each separate wheel texture, but you will need a spec map to get the colour right. The spec map doesn't need to be overly high def; I actually resized it down one level (512x256 instead of 1024x512). Also, this spec map has the alpha removed (the template will already apply the alpha).

The download:
http://www.mediafire.com/?1cpbd4sb41d422s