Tire calculators

The OE tires were 175X14. Aspect ratio was not yet in use but I believe they were approx. 80-83. The overall diameter was approx. 25" so if your new shoes stay close to that number you will avoid speedometer error.

11 minutes ago, jfa.series1 said:

The OE tires were 175X14. Aspect ratio was not yet in use but I believe they were approx. 80-83. The overall diameter was approx. 25" so if your new shoes stay close to that number you will avoid speedometer error.

185/ 75 R 14 is 24.9"

195/75 R14 is 25.5"

195/70 R14 is 24.7"

I like this one - though not a calculator, the page has many combinations and how they compare to stock:
https://www.automobile-catalog.com/tire/1971/2104115/datsun_240_z.html#gsc.tab=0