[Vwdiesel] 13' and 14' wheels

[Rudy]

I have an 85 Jetta with 175 70R 13 tires and wheels and a 91 Jetta with 185
65R 14 wheels and tires.  I'd like to swap all four from the 91 to the 85.
 
Can someone tell me how to calculate the difference between the diameter and
circumference of the two wheels and tires?  Also does anyone know what year
VW changed from 13 to 14 inch wheels on the Jetta?
 
The 91 is not running and the 85 is and needs tires.  
 
Rudy