Assuming the 80 inch is the outside measurement and not the internal bore size.
Formula is Circumference = radius squared timed by pi (3.1416)
40 × 40 × 3.1416 = 5026.56 inches
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Diameter = 80/pi feet
80 pi inches
If circumference = 160', and circumference = 2 x pi x r, then... 160 = 2 x pi x r 80 = pi x r 80/pi = r 2(80/pi) = diameter = 50.99'
divide it by three. that gets it close.
80 if the tiles are 12x12 - 12x12 is one square foot covering 80 you would need 80 tiles