With me this issue exist with models downloaded from 3DWarehouse to SketcUp and then exported SH3D.
Usually method alaX suggsted works fine and removes those typically gray patches from renderings.
Bit more still..
Original picture was not available, so I used Hummer-no-black.zip
Rendering show gray patches in tires and they disappear when 'material_5' is marked invisible.
Left Hummer intact, right one stripped to show that tires are OK and some other thing.
Interesting detail is seen in 3D-view..
Rendered image below
3D-view screen capture below
That detail: marking material_5 inivisible make wheel hubs transparent in 3D-view but just when looked 'outside'. And rendering above show hubs black/dark gray as designer intended. Interesting.
A bit long story to notice that imported things can contain object that is transparent from other side and completely opaque when looked from other side.
Anyway: now is Saturday evening, time for sauna and beer after that.
Everything begin to feel much better!
