Graeme. I would say it depends on what size tyres they hung off the boat. A 16 inch car tyre is only about 13mm in this scale, and so working upwards a 25mm size tyre would be a real 30 inch tyre (I think I have done the maths right) .
HOWEVER, what looks right on the boat is a different matter and I would say that an approximate size could be obtained by looking at pictures and measuring up the appropriate visual clues on your boat and going from there.
Do we think that Graupner chose the tyres on the basis of the correct size tyre as fitted to the ship (at any one time of course), or they used commercially available model tyres that looked OK ??
I apologise to the purists here, but its the way I would go.
Ashley