Circumference will be the same, and only slightly off if you chose a different side profile. But the Width doesn't affect the circumference.
The diameter of the wheel is 20" The width of the wheel will be somewhere between 9-12".
If you don't want the sidewall of the tire to extend past the face of the wheel, then take the width of your wheel and do a standard to metric conversion to find out the closest width in mm's. Then get that width of tire.
Does that help?
