As the tire deflates, the distance from the center of the wheel to the road surface(the actual radius of the circle) is reduced, thus creating a new "true" circle. The tire will be(and always is) a different shape when weight is applied to it, but that is immaterial, so the statement in the earlier post is correct. The ABS measures wheel speed and the tire with lower pressure will have a greater wheel speed than the other one. There is a range of allowable difference so that slight variations wouldn't keep flashing the warning. From what I have experienced, the tire has to be mighty low to trigger this warning.

Doug
'99 Black GP GTP Coupe