" I’ve created all the wheel, excepting the tire" should be “I’ve created the whole wheel, except the tire.” - “All the wheel” isn’t right unless it’s “All the Wheels” and drop the ing off excepting.
As far as your problem, this is the most common question of all beginners I think, it’s easy to fix though.
Go into your edit buttons and you’ll see a box that says 1 Mat 1. This means one material index out of 1. If you want more than one material on the same mesh, you need two or more material indexes. So hit New and make it 2 mat 2, then select the faces you want different from the rest of the wheel and hit Assign. Now go into your materials, and you’ll see the number 2 next to the box where it says MA:Material. This means that MA:Material is a material that is assigned to 2 different material indexes, 1 Mat 2 and 2 Mat 2. So click on the number 2 and make it a single user, this gives 2 Mat 2 it’s own personal material.
Switch between the two materials by clicking on the 2 Mat 2 there in materials.