Hi Rog,
I have seen that calculator, unfortunately its the physical sizing up, offset of the rim, OD of the tyre and width that will fit well into the guard that concerns me. And it seems nobody can really help with it
Reason for 15" is I personally love the look of the old school 'fat' tyres. I think they look great on the cobra so want to persist with it.
I have already researched tyres quite a lot, and although the options are not huge, there are a number of brands I can get here or brought in.
Just amazed there are no DRB boss cobra guys on this forum that can offer what tyre size they are running