I recently wanted a set of 175 R 13 tyres for a 1970 Triumph 2000, which was a relatively common size back in their day. They are still available, but only from a few manufacturers as a 175 80 13, at a cost. I also wanted a set of 185 R 13's for a 1972 2.5PI, which have been virtually obsolete as a car tyre for years, although available in a van tyre with stiffer sidewalls making the tyre less flexible and this affected the ride and handling. Good old Maxxis Tyres have started making both sizes in a car tyre, and I am very impressed with their ride quality, appearance, and and price. Only downside is the thin white wall, but as they can be mounted either way, it can be put on the inside with only the MOT man ever seeing them (if I choose to have them MOT'd that is....)