User blog comment:Xxkiiroxx/"Shingeki no Kyojin is Set in North America" Theory/@comment-108.11.137.243-20141011140208

Um, you made a mistake with the walls' dimensions

http://shingekinokyojin.wikia.com/wiki/Walls

It says here that the total "diameter" (not the radius) of the walls is 480 km, since Wall Sina's diameter is 250 km and the space between the walls is 130 km (Sina-Rose)  and 100 km (Rose-Maria). 250 + 130 + 100 = 480 km for the diameter.