The Circumference of the Earth Puzzle

The answer is "just over 3 feet."

The formula for the circumference of a circle is PI * D where PI is 3.141592654 and D is the diameter of the circle. Thus, no matter what the original diameter, the change in circumference is equal to PI times the change in the diameter, or, for every foot the diameter increases, the circumference increases 3.141592654 feet.