Tire diameter calculations:
Check your tire number:
For example, 205 75 R15:
205mm=20.5cm=8.07inch (Tire Width)
75=75% (Flat Rate)
R15=15inch (the Diameter of Rim)
Tire Diameter= 8.07x75%x2+15=27.1inch.
And you can choose the cover size from 27" to 29".