New answers tagged transparency
For the sake of easiness, use GLSL shaders. This is mandatory for modern openGL, anyway. Now, what you have to do is to unwrap your object to each texture. So you must associate to each vertex two texture coordinates in your vertex shader. Plus, you may wish to add a [0-1] value X to each vertex that tells how much of the first and second texture you ...
According to the way the documentation lays out the single alpha channel for both specular and transparency you are correct in your assumption. If you want full control over the textures my solution would be to separate your tire and rim-hubcap models so you can texture each individually. This would allow you to create a Transparent Bumped Specular on the ...
.png images wont show the transparent area. But you will have to make sure that the "white" area is transparent and not white. If its not a .png image just go in to photoshop or any other image editor, cut out (magic wand) the white area and save it as a .png
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