(222 * 0.75 / 25.4) * 2 + 15 = Tyre Diameter
222 in your example is the width, 75 is the percentage of the width that is the sidewall height (lower number less height in sidewall), divide this by 25.4 to get it in inches then multiply by 2 to get the amount of tyre height on the rim and add the rim height, 15" in this case.
So this one is 28.1 inches tall and 8.74 inches wide (222/25.4)
To get a 33 you'd need something like 305/75/15. that would be 33x12x15
Easy
Cheers
Steve