And if my work isn't good enough, let's see what Bridgestone/Firestone get when they do the same math.
http://www.bridgestonetire.com/dpp/sizespecs.asp?passproductid=227
215/55-16
Overall Tire Diameter = 25.3"
Static Load Radius = 11.6"
225/50-16
Overall Tire Diameter = 24.9"
Static Load Radius = 11.5"
Seems Bridgestone/Firestone agrees with me on this one.
Remember I got 25.31 (so they rounded to 25.3") for the 215/55 and I got 24.85 (and they rounded up to 24.9") for the 225/50.
So now you have to convince the world that both me and Bridgestone/Firestone don't know how to measure tires.
And I'm sure I can find another tire maker and look at his products as well, and find nearly the same numbers.
http://www.goodyeartires.com/catalog/products/EAGLERSASize.html
FWIW,
TB