http://tf.llu.lv/conference/proceedings2011/Papers/041_Biris.pdf

The inflation pressure acts perpendicular(follower force) to its inner surface.

A constant surface force is added to every element at the inner surface of the tire and the direction of this force is kept perpendicular to the surface during each solution increment.

http://tf.llu.lv/conference/proceedings2011/Papers/041_Biris.pdf

Hi

As I Know in AnsysWorkbench :

you must choose a surface that include all inner surfaces in your Tyre model then apply a pressure load on it.

if your geometry is built from more than one surface you can merge them virtually in virtual topology option in mechanical to have unique surface and apply load.(I recommend this.)