Coldwave, the details like the bumps on the wheels and the stuff on the engine could only be achieved by displacement maps (with the addition of normal maps, but i don't know if normal maps show up in AO render) if not actual geometry. the purpose of displacement maps is to displace the surface, bump and normal just give the illusion

you could add all that in something like z or mudbox or you could just make the higher poly version and then use transfer maps

as for the triangle holes in the wheels, it might have been some transparency map, but i thought that would just make the surface transparent in places, the inside of the wheels have depth... i don't know how you get that... so i can't offer anything there...