Hi Lunatek,
there are different tyres availible by Roco. Just take a look at you local modelrailroad dealer.
I suggest you to buy a 4- axle military-truck with the tyres you wish. I think the heavy trucks comes with the tyres you searching.
Maybee there is also a sparepart-box with tyres availible.
The herpa tyres are a little bit smaller. If you want to use this, you have to change the complete wheels.
Sometimes it looks better to change the herpa wheel into the roco wheels, so you it looks like a little "monstertruck" ;-)
Rubbertyres are also availible at (surely an incomplete list):
trident (mostly onroad wheels availibe as ad on)
Faller (spare parts of the "Faller Car System)
Herpa (availibe as "Offroadtyres" ad on)
Roco (mostly the trucks come out with rubbertyres)
Brekina (old time onroad wheels)
Busch (there are a lot of different rubbertyres in a box availible to put it in a railroadwaggon as charge)
best regards
Thorsten
www.mikromodell.de