The I-5 originally used 13 inch car tires and rims. The rims were off of early 64 1/2 Ford Mustangs and Econoline vans. Good luck finding them! They were also used however on many camping trailers of the day and brand new reproductions are available:
http://www.etrailer.com/Tires-and-Wheel ... 20214.html
The original tire size is B78-13 which converts to a 175/80/13
These are the replica rims riding on new old stock Armstrong Norseman tires.