Spare Tire Cover

[fast150]

I recently put 550R19 tires on my 31 Tudor. Now my spare tire cover won't fit because of the larger tires. I would like to know if anyone has tried the 20" cover Snyders sells, or if I need to go with a 21". The mounted tire measures 30" diameter to the tread corners, so I'm leaning toward the 21". I just don't want to spend $50 for a cover that doesn't fit.

19 + 5.5 + 5.5 = 30.
21 + 4.5 + 4.5 = 30.

This is my dilemma.

Thanks for your help.