Caz (with one zed)
Oh bugger I've just written war and peace on this, previewed it but not sent it.
Lucy you, you now get the abridged (slightly) version of this.
Small Dandys = 10" wheels 6's = 12"
Look at [You must be registered and logged in to see this link.]
Then download the original brochure (Help tech types how do I attach or host a PDF file on this site?)
Look it has the wheel and tyre size 12" and 155SR 12
The things you need to know about now are: PCD. Centre Hole Diameter. Offsets. Load Indexes. Tyre Speed Codes. Aspect Ratios. Stud Diameter.Wheel Nut Profile.
This can be circumvented by calling Ian at Riva [You must be registered and logged in to see this link.]
and he will give you the dimensions for the wheel. Or more usefully tell you what vehicle the wheel was intended for i.e. the small Dandys have mini wheels.
I see Mike has already corresponded this afternoon. Give him a nudge as he has a 6 so probably knows most of the details for tyres off the top of his head. Well either that or senility has struck and that is why he has not posted you already. (Senility says the the man who forgot to send his own post)
Tyres: the 155SR (Steel Radial?) is an obsolete form of nomenclature but I seem to remember old radials having an aspect ratio of 80% the only 155 X 12" that I can see are no higher than 70%. This might loose you 15mm or so in height and wouldn't match the original tyres. If the originals are still on we will be replacing them WONT WE 20 years+ is a bit much.
Look at this ebay advert [You must be registered and logged in to see this link.]
I doubt that this wheel and tyre combination is right for you but if you scroll the advert down you will see a number of guides that explain the terms I used above. This is really helpful stuff and it's a better job than I could do.
Things not mentioned are that caravaner's sometimes choose tyres of greater ply rating as standing still can be a hard life on the sidewalls. Truck or commercial tyres with 8 or 10 ply tyres, how relevant this is to a larger Dandy .....? Ask Mike or see what Ian says was fitted originally.
One issue that should not affect you is that a lot of the 10" trailer tyres you see are "Bias Belted" this is almost like crossplys and if the 1960 after Caz referred to your date of birth you might just remember them.
Hopefully not overawed you, those guides do explain things, and given you suitable shortcuts to get the information. If you don't understand something or I've not explained it properly (more likely) post again.