Originally Posted by FBinNY

Bicycle tires, like any flexible tube will form a near perfect circle when inflated. The basic physics is that shapes under internal fluid pressure will deflect to maximize volume for the perimeter (circumference).

Of course this depends in the structural factors that would resist that tendency, so won't apply near the rim.

In the case of bike tires, with supple bias ply sidewalls, the resisrance is near zero compared to the high inflation pressure.

Note that the circle is at the wall, so different tread and wall thickness will cause the outpear more oval.