Hi Jimbob,
there is a ROCO spare kit "Tyres" availible. Inside youll find 2 kinds of rubber tyres. The smaller one is good for the unimog.
Another hint. Herpa sales "Offroad Truck wheels" in a small box. They comes out including rubber tyres, wheels and axles. This one also good for the ROCO unimog.
As the last way I suggess you: You can also make tyres by your own.
Take a plastic-tyre in a shape
cant some silicon-rubber inside the shape
So you have a negativ-shape of the tyre
Now you cant latx-rubber inside the shape. After some hours the latex is dry and you can take a wonderfull tyre out of the shape.
(The secret is: silicon and latex dont glue at each other)
Maybee the Roco or Herpa Sparekit is the easier way ;-)
best regards
Thorsten
www.mikromodell.de