A recent quest for the very rare 155/80/R12s that my Hijet uses made me consider the alternatives. Apart from Ivor Williams, not many manufacturers us 12" wheels these days so converting to larger rims looks attractive. There's loads of relevant info over at:

http://www.carbibles.com/tire_bible.html

On page 2 there is a calculator that lets you compare the rolling circumference of different combinations. So I set about finding what tyres are commonly available. There seems to be very little standardization so just about any combination is possible - even though it may not be readily available.

Anyway, here are the results of my research. The column labeled 'Availablity' is just a subjective assessment based on the list of tyres presented by blackcircles.com

I hope this info may be useful to anyone considering new alloys. But note that I have not tried a 195 tyre so, although it might look nice, I've no idea if it would actually fit.

Width Prof. Rim Circ. Error Avail.
155 80 12 1737 0.00 poor

155 70 13 1719 -1.01 good
155 75 13 1768 1.79 none
165 65 13 1711 -1.47 good
165 70 13 1763 1.52 excellent
175 60 13 1697 -2.28 poor
175 65 13 1752 0.89 moderate
175 70 13 1807 4.05 excellent
185 60 13 1735 -0.11 moderate
185 65 13 1793 3.24 none

165 55 14 1687 -2.84 none
165 60 14 1739 0.14 good
165 65 14 1791 3.13 good
175 55 14 1722 -0.85 none
175 60 14 1777 2.32 moderate
185 50 14 1698 -2.21 v. poor
185 55 14 1756 1.14 good

175 45 15 1692 -2.59 none
175 50 15 1747 0.58 poor
175 55 15 1802 3.74 moderate
185 45 15 1720 -0.96 none
185 50 15 1778 2.39 none
195 40 15 1687 -2.86 none
195 45 15 1748 0.67 good
195 50 15 1810 4.20 excellent

185 40 16 1742 0.29 none
195 35 16 1706 -1.79 none
195 40 16 1767 1.74 poor
195 45 16 1828 5.26 good

195 30 17 1724 -0.72 none
195 35 17 1785 2.80 none