[CapriRacer]

A 1966 Plymouth Valiant originally came with either 6.50-13's or 7.00-13 bias tires on 4.5" rims. It sounds like you are not willing to pay extra to get authentic reproductions - which is the only source for tires in those sizes.

The closest modern equivalent is P165/80R13 and P185/80R13, respectively. Unfortunately, tires are no longer made in those sizes either - except for the afore mentioned authentic reproductions.

However, a P175/70R13 or P185/70R13 can be used on this vehicle. There aren't a lot of 13" tires being made, as they also are being phased out, so the selections are pretty slim. Just be aware that the diameter of these tires is smaller than the original and the speedometer is going to read low.