The original tires are 215/75 R-14, but those are (apparently) no longer available. Went to Discount Tires and they looked up a replacement tire size. Their online guide said to use 215/70 R-14.

Due to the slight difference in size, this will cause the speedometer to read incorrectly. If the speedometer reads 65 MPH, the approximate actual speed will be 64.27 MPH. I think I can live with that slight difference.