The ST145/R12 have a diameter of about 21.3". For a tire cover I recommend the 290-3975 Adco Ultra Tyre Gard # 290-3975. These are very solid vinyl covers which will keep your tires protected while in storage. They will fit virtually any tire between 18"-22" in diameter, including your 145x12 .
I have linked our review/install video below if you want to take a look. view full answer...