If you go to sites where you have to haul the Dandy over grass by hand, having an air filled tyre on the jockey wheel really helps, especially after rain when the grass is soft. It is also good if the ground is bumpy.
They seem to come with a 48mm diameter. On my '85 4/5 I had to fit a new clamp to accommodate this, but when I switched it over to my '88 4, the original clamp was already the correct size. If you are switching anyway, the extra cost for the pneumatic version is not that much. Mine was about £30 with the clamp.
There is a thread on this forum about them [You must be registered and logged in to see this link.]
which also contains a link to the one I bought.
and this is a picture from that thread showing the wheel and clearance once fitted.[You must be registered and logged in to see this image.]