[P-hack]

What capri is saying makes perfect sense, that load/pressure (and speed, as the weight of the tread pushes outward) affect the roundness. I think aspect ratio would have an effect too. The steel belt only limits the maximum circumference, it can be distorted locally.

Nit pick, it looks like with 7 feet lining up with 2 inches, that would be 82 inch diameter, for 96.4%

But the tape measure on the floor is showing 82 1/8", so unless I'm missing something, it sounds like the same diameter loaded and unloaded :/ (perhaps more than a nit)