My 1975 Ghia came with CR70-13 tires. Rims are 13 x 5 with 4 bolt pattern. The Mustang specs do not list a 14" option for 1975. I have found a set of 14" x 6" chrome rims from a 1979 Mustang. Will they fit and what size tire do I use to make sure I don't screw up the speedometer readings.??
To keep the speedometer reading the same, you need a tire with the same overall diameter or same revolutions per mile. I couldn't find size data on the original, but MVMA specs lists all tires for the 75 II at 888 revs/mi. I don't think that's quite right (195/70-13 should be about 849 revs/mi), but the speedo is probably set for that.
The closest I could find to 888 revs/mi in a metric 13" is the 175/70-13 at 891. The closest in 14 would be:
185/60-14 (887revs/mi)
205/55-14 (882revs/mi)
225/50-14 (882revs/mi).
Keep in mind that with taller tires, you're going to be faster than the speedo indicates.